Part of the issue in answering this is that there isn't really an answer. At best we can say there are shades of acceptability that have shifted over time, and at least in some senses, we still have not reached a point where we can decisively say it is unacceptable, as there are examples which could be pointed to which nevertheless break the 20 year rule.
That doesn't mean there aren't major points we can look to though in the evolution of the unacceptability! To be clear, there are multiple ones to concentrate, and I am not looking at all of them, but the span of time I'm looking at is nevertheless an important one:
ARTICLE I
The High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another.
ARTICLE II
The High Contracting Parties agree that the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means.
ARTICLE III
The present Treaty shall be ratified by the High Contracting Parties named in the Preamble in accordance with their respective constitutional requirements, and shall take effect as between them as soon as all their several instruments of ratification shall have been deposited at Washington.
While it now, if anything, is the butt of jokes, the 1928 Kellog-Briand Pact is actually more deserving of respect than it ends up getting in popular memory. Signed in 1928 as an attempt to 'Renounce War' as 'an Instrument of National Policy' by the great powers of the world, as its name states, it seems quite laughable in hindsight thanks to that minor conflict that erupted a decade later known as World War II, and which saw signatories such as Germany and Japan involved as aggressors. And to be sure, it isn't an unfair criticism! Outlawing war seems like a fools errand even today, where perhaps we have avoided great power conflicts in our lifetime, but certainly have had no end to smaller outbreaks of interstate violence.
But nevertheless, the Kellog-Briand Pact does represent an aspirational turning point, and an expression in international diplomacy of the unacceptability of armed conflict as a means to solving international disputes in a way that surpassed previous international agreements. More importantly though, it provides a binding expression of international law. To be sure, the treaty was entirely toothless. The three scant articles were entirely aspirational and included no method for enforcement, and did nothing, in the end, to prevent the Second World War.
What they did do though is nevertheless quite important! After the Allied victory over the Axis, trials were conducted at Nuremberg and Tokyo, and charged, among other things, with crimes against peace, a charge which they were able to substantiate with the signatures of the Axis power to the Kellog-Briand Pact, wherein they had renounced war of aggression. It should be noted that there was considerable debate over whether the pact provided sufficient justification, due to its lack of an enforcement clause, but in the end it was decided that enforcement was justifiable, with a key argument being that the Hague Conventions, which laid down the basic laws of war, likewise lacked enforcement mechanisms for its various clauses, yet had nevertheless been treated as such. The specific treaty having an enforcement mechanism was less important that its validation of the principle as a part of positive international law.
Once the contentious issue of whether the charges could be brought in the first place was settled, it was an incredibly easy charge to prove and convict for. The guilt of the accused, was all but self-evident, the only serious issue to establish being whether the treaty itself meant anything. The result then at Nuremberg, and the less remembered Tokyo Trials, was the enforcement of the spirit of the Kellog-Briand Pact, and giving meaning to its lofty ideals! At no point prior in history can we point to a situation which is so simple as to accurately summarize that 'War was outlawed, and violators convicted'.
But... let us not get ahead of ourselves and spike the football. One of the prosecuting powers was quite guilty themselves as whatever their protestations, the Soviet Union had sliced up Poland with the Germans back in 1939, and plenty more examples of war as 'an Instrument of National Policy' dot the post-World War II historical record, not only without prosecution, but in some cases conducted by the powers who had engaged in the prosecution that established the precedent in international law.
The Kellog-Briand Pact, nominally at least, remains in effect. Likewise so do the precedents that resulted from conviction at Nuremberg. Inarguably, attitudes did change, and creation of the UN, whose charter provides similar sentiments to the Pact, helped to further reinforce them. At least occasionally the UN's charter even provides legal backing to intercession against violations of these principles, such as in Korea, which was fought under a United Nations flag, or Resolution 678 which authorized the use of force against Iraq in 1991. But we return to the beginning as well and must reemphasize once again that we can only talk of shades of acceptability. The mid-century period surrounding the Second World War was a critical junction, and perhaps the single most important shift insofar as we look at relationships between the 'great powers' and their use of military force against each other, but its impact on smaller powers, let alone great powers in relation to smaller countries is very much a changing shade at best, with numerous examples easy enough to point to as at least worth discussion as to whether they violate those principles, and which violate the 20 year rule but are no doubt easy enough to deduce.
It is important not to go so far here as to argue simply that 'might makes right', but it certainly is important to emphasize that the strictures of international law are not ones which can be enforced uniformly. Does this mean we can't say norms concerning what was and wasn't acceptable in terms of aggressive war and conquest shifted through the 20th century? Of course not! But it does mean we must be cautious in talking about absolutes, or of seeing changes in too emphatic a light. We can look both before and after the period being concentrated on here and see other critical junctures in the shifting attitudes towards acceptability, but that itself speaks to why it can be tough to look at this question in anything other than shifting shades, rather than finding one, single turning point.
Sources
Hathaway, Oona A. & Scott J. Shapiro. The Internationalists: How a Radical Plan to Outlaw War Remade the World. Simon and Schuster, Sep 2017.
Hirsch, Francine. Soviet Judgment at Nuremberg: A New History of the International Military Tribunal After World War II. Oxford University Press, 2020.
Plesch, Dan. Human Rights After Hitler: The Lost History of Prosecuting Axis War Crimes. Georgetown University Press, 2017.
Krammer, Arnold. War Crimes, Genocide, and the Law: A Reference Handbook. ABC-CLIO, 2012.
Fairly recently, to the extent that this question is what a number of articles posted to the sleepy subreddit r/IRStudies try to answer in a slightly altered form: what drives peace, and why does it seem that in the past seventy or so years, the international system has largely peaceful, devoid of wars of conquest as indeed you ask?
Is this even a History question, or is this an International Relations question? I will say that I believe this question is unanswerable from a historian’s perspective: What constitutes a conquest across societies through history? What does it mean when a conquest is “Acceptable?” (I would argue that a conquest is never acceptable in the eyes of the conquered) In antiquity, across Europe and Asia, when nomadic peoples raided and extracted tribute from more stationary societies, was this conquest? What of the Empires which relied off of tribute extracted from peripheral peoples in order to enrich the homeland? Did the ancient empires understand conquest as we do, given the emergence of concepts of “Universal Empire” (present in some form both Ancient Rome and China)? If we want to fish for a specific example, when in the Early Modern Era the Kings of France or Spain sliced off the ruling dynasties of Italian states like Milan and Naples without laying hand on the underlying institutions, was this really a conquest? These are all questions which can be answered in a variety of ways, and these answers will have a different impact in our response. Building on my last example, I could offer a multi-part answer exclusively on the French and Spanish "conquest" of Italy and we would still need to dig deeper into why or how decision-makers found that conflict "acceptable" (or "unacceptable").
So what I'm getting at is that there is no “real” answer to your question, but I can try to first inject some nuance in the notion of the historic “acceptability” of war in the western european history, and then veer onto a conclusion that will necessarily look at how an answer to this question might differ between various traditions of international relations theory. I'd also point out that anyone who might want to reply or offer their own answer from other perspectives is of course welcome to do so (I only point this out as the question has gotten a lot of upvotes, and I wouldn’t want my reply to be taken as the only possible response). There might, for example, be a parallel answer to be had as pertains to the history of philosophy on the legitimacy of war and conquest (be it philosophy in the Eastern, Western, or other world traditions).
As pertains to modern notions of statehood and sovereignty, in the western tradition these ideas are generally said to have been first codified in the the Treaty of Westphalia at the end of the Thirty Years War. We can choose this point in time as the convenient first signal where the high politics of Europe sought to begin codifying what constituted acceptable interactions between sovereign states. But does this mean that the multitude of princes and diplomats, as well as the Venetian mediators, present in Osnabruk and Munster collectively had no notion of needing to "Legitimize" wars before this point?
Not really. Historians stay away from generalizations, but one that I am confident in making is that war is always a costly, disruptive, and highly destructive endeavor, which meant that long before the Treaty of Westphalia was signed, even the most authoritative ruler was accustomed to expending energy to galvanize subjects into accepting (and participating in) the business of war (and this is true today as it was in the past — but spending too much time on this would be veering too far into the real of modern social science and away from history). From the descent of Emperor Frederick (the “Barbarossa”) into Italy, to Charles VIII of France sparking the Italian wars, serious time and energy was expended to justify and legitimize the undertaking of war (I hope you will forgive my natural tendency to skewer examples towards Italy and the Mediterranean). My point is that war, especially war for conquest, is not exactly a natural concern of the state under any definition of the social contract; the very act of embarking on war is itself delegitimizing. Therefore, the right to rule by inheritance or acclamation, the real or perceived need to halt a growing threat or stabilize a border, the exploitation of religious or historic differences (or similarities), and good old opportunism, were just some of the reasons periodically produced in order to legitimize a war to stakeholders (often in a variety of combinations). Sure, there were no mass propaganda drives and elaborate mechanisms of enfranchisement that are necessary to legitimize modern conflicts, but a restricted circle of the empowered does not mean the absence of stakeholders altogether. And even pre-modern stakeholders knew full well that war was destructive and damaging, in no way guaranteed to be offset by plunder, perceived glory, and conquests: in the aforementioned example of Emperor Frederick, the highly destructive bouts of fighting in Northern Italy he pursued not only had the effect of consuming the very resources he was hoping to lay claim on, but also weakened his position relative to his subjects in Germany, and also imparted an intergenerational scar on communities in Italy which would forever change their attitudes to government and power, especially imperial power. But the Emperor Frederick himself was only building on the canons set by his ancestor, Emperor Otto, who at a critical moment in western european history had felt more much more legitimized in strengthening the imperial bonds between Germany and Italy and not, say, between Germany and France. For every european war of conquest, we can find specific and unique reasons why its participants felt that what they were doing was indeed, “Acceptable.” If they didn’t, the war wouldn’t have happened. In my own area of expertise, I could delve into how even the most opportunistic and seemingly unjustified conflicts, such as the 13th century Sack of Constantinople, were perceived as perfectly legitimate by the people who triggered them.
That is not to say pre-modern Europe was a peaceful place save for a few unique exceptions. Flimsy pretexts for the powerful to coerce the weak abounded. Opportunism was rife. Pretexts which today would be unacceptable, such as religion or colonialism, were instead seen as perfectly acceptable. But my point is that war almost never emerged exclusively for conquest’s sake. To the people involved, all the reasons listed above represented very serious justifications for conflict. And although I am focusing on pre-Westphalian Europe, post-Westphalian Europe was similarly constrained. Even at the height of the 19th century colonialism, justifications ranging from resource extraction, to a "Civilizing Mission," to the need to defend the metropole by projecting power across the world, necessarily emerged to justify the colonization process. Likewise, the innumerable causes of First World War, and the iconic (for all the wrong reasons) propaganda and nationalism in the lead-up to the Second World War, coupled with the very fact that both conquests erupted into world conflicts amid a narrative of curtailing an expansive aggressor, indicate to us that the "acceptability" of these conflicts was far from a simple foregone conclusion.
Does this mean my long-awaited answer to the question is that the roster of justifications for war has shrunk in the past seventy years? Maybe. I'll get into more detail in the second part to my question below, but we will have to veer into the world of International Relations theory.
Why? I've answered questions comparing International Relations methodology to Historical approaches in the past which might interest you. In short, the fundamental difference is that while history is a liberal art which looks to examine and deliver insight into the causes and consequences of past events, international relations theory is instead a social science which seeks to deliver systematize truths. In other words, a historian won't have much to say about the concept of "Open Conquest" which you ask about, however they might have a lot to say about the justifications and drivers of a specific conquest (that the historian happens to know something about). An International Relations theorist, on the other hand, will have an enormous amount of things to say on your question on "Open Conquest," even if they might not be entirely well-versed in the details of a specific conflict (or conquests, as in our case).
Understanding when, and equally useful, why, conquest came to be unacceptable internationally hinges on a few things. What do we mean by ‘conquest’, crucially, what constitutes a “nation”? To separate these from other territorial arrangements from war (like military occupation), here’s the Permanent Court of International Justice definition, at a time where the League of Nations was heavily preoccupied with the issue: conquest is a “‘a cause of loss of sovereignty when there is war between two States and by reason of the defeat of one of them sovereignty over territory passes from the loser to the victorious State”.
A bit of a mouthful (and we need to unpack what sovereignty means), but I think it suffices here. But even when norms against conquest spread, the provenance of conquest, as linked to international law, might imply there are some forms of aggression that did not receive the same scrutiny, especially when who received recognition of a nation, with all the benefits of this status, was historically conditioned. This brings us questions on what the unacceptability of conquest demands, beyond responding to an ongoing act of conquest: the reversal of conquest itself?
In other words, you’ve asked a complicated question (as the 100+ deleted comments here suggest). I suspect the easiest answer to give you would be to trace the discussion of ‘conquest’ throughout international law, which usually leads to an account of some older, if not ancient custom of war that established a right to conquest – in Roman times, the medieval period, and into the 16th century with Hugo Grotius, Kant and other usual suspects. Various developments like the 1713 Treaty of Utrecht in Europe seem to constrain, but not rule out conquest, in making conquest’s legitimacy contingent on a peace treaty – although there’s some legal debate over whether that meant the peace treaty (not the conquest itself) was the basis for the transfer of territory. I don’t feel comfortable giving a precise rundown of this period, other than to say this account largely privileges a Western, and increasingly rationalist, account of international law. This usually risks overstating the extent to which law operated independently of politics - the evolution of international law as we know it today is fairly recent.
Nonetheless, by the 19th century, the validity of conquest began to be contested more and more: the Monroe doctrine was intended to avoid territorial changes in the region arising out of coercion and war, leading to the declaration at the First Inter-American Conference (1890) that “the principle of conquest shall not…be recognized as admissible under American public law”. Emphasis mine – this nonrecognition of conquest was construed as a domestic development, although other similar treaties like the Saavedra Lamas Treaty (1933) signed in Argentina echo this.
From here, determining when conquest stopped being internationally acceptable is a trickier question, especially defining ‘international recognition’ or ‘validity’ requires dealing with who the international community was. The League of Nations certainly played a major role, especially in Article 10 (“The Members of the League undertake to respect and preserve as against external aggression the territorial integrity and existing political independence of all Members of the League”). But for the League, in 1924 the question of how to respond to such aggression was left open to the Council, owing to the political impossibility of specifying action within its foundational document.
To continue sketching this rough timeline, the two-article Kellogg-Briand Pact (1928) further “condemn recourse to war for the solution of international controversies and renounce it as an instrument of national policy in their relations with one another” in its brevity, which in effect would have condemned Japan’s conquest of Manchuria, or Italy and Ethiopia, which occurred anyway. Nonacceptability did not mean effectively translate to prohibition. To state the obvious: there is no authority analogous to what states have over us, that exists in the international sphere (‘anarchy’, the International Relations literature suggests).
Yet the nonexistence of a sovereign capable of using coercive force against states not complying with international agreements or behaving aggressively does not mean no form of higher political authority exists. International attitudes against conquest were – and are – always evolving, and I’d point to the United Nations, prohibiting force or the threat of force (in Article 2) as one culmination. The UN does not write international law, of course, but is comprised of its member-states and an institutional, administrative body (the UN Secretariat), organized into various agencies and councils. But through its resolutions, the UN has both affirmed certain norms against conquest and aggression, while developing various processes and institutions intended to contain attempts at conquest.
Most visible amongst these is peacekeeping, which developed initially as an extremely limited force intended to observe, not involve themselves, in conflicts – as the name United Nations Emergency Force implies, when it was deployed in the Suez Crisis. Peacekeeping is especially notable as a response to conquest given that the UN never realized its plans for a standing army, something specified in its Charter as well: it implies again the political, diplomatic difficulties post-WWII in cooperation that posed limitations to attempts at international cooperation, and focusing solely on formal UN instruments and documents cannot tell us much about the complex nature of international politics going on elsewhere.
But even the General Assembly, for all its pretenses to be universal, was never as general as suggested – in the UN’s first decades, various colonies did not receive recognition. Were these nations? For all that nationhood implied a right to self-determination, to be exercised when they were not self-governing due to colonization or conquest, the situation of nations like Tibet never received formal attention at the UN, which was largely preoccupied with the Korean War. The People’s Republic of China took its seat at the Security Council in 1971, effectively ruling out this possibility. There have been numerous resolutions that have since outlawed, or affirmed the outlaw of conquest – often in specific cases like in the former Yugoslavia, Nagorno-Karabakh, or Georgia.
However, historical and political terms like the (legitimate) “nation” cannot escape more normative judgments. Beyond questions of what counts as a nation, whose free status and rights to self-determination would have to be preserved, what should be done when this peace was violated? Peacekeeping, as the name implies, arises largely when there was a peace to keep, and even longstanding operations like in Cyprus ostensibly prevented conquest of the island by one party or another, without necessarily delivering a political solution. The same might go for Israel, the context I suspect you had in mind asking this question, where UN resolutions in 1968 essentially denied the validity of conquest or boundary changes, a position incongruent with territorial changes and Israeli occupation since. (I’ll set aside discussion of more recent resolutions given the sub). Most dramatically, attempts by Iraq to conquer and annex Kuwait were met with the Security Council’s authorization of an intervention. But even this further complicated matters when UN authorization was involved a decade later to justify war in Iraq (again, 20 year rule!)
Here, the political science and IR literature has something to say too: Douglas Lemke has an interesting article arguing that norms against conquest seem to hold only when all parties are recognized as de jure sovereign states, but less so for nonsovereign polities (think Somaliland – whose peaceful behaviour seems to qualify them for membership under UN). Yet his categories and descriptions of conquest remind us that what categories we use to analyse and historically evaluate conflicts and politics matter: unlike Lemke, I don’t think the Rwandan Patriotic Front defeating the Hutu government in Kigali to take power was a “conquest by territorial contenders), nor was there a “conquest of the Tamil de facto state” in Sri Lanka – situations better viewed as a civil war. Nonetheless, if the UN, alongside a non-comprehensive system of international law is the closest there exists today to some form of international political authority, my answer to your question would be that it certainly became unacceptable to engage in conquest by 1945.
I’ve offered only a sketch on how we might think about conquest and the hardening international norms against conquest here. What I’d like to reiterate, however, is that this question shouldn’t be considered as a purely legal one, or independent of what responses came after – just like how a criminal code not specifying the punishment would be vague at best, insidious at worst.