hi all. Just a reminder to potential respondents that current events are off limits in this sub, per the "20 year rule": do restrict all discussion to events/conditions up to 1995 only. Resist the urge to respond to the OP's allusions to more recent events. Thanks!

Sweet, an answer I can (probably) contribute to!

So firstly, to validate OP’s sense that in modern times conquest has been delegitimized, this trend has been noted by political scientists and termed the “norm against conquest”. Mark Zacher and Tanisha Fazal are two such scholars who have written about trends, attitudes and changes in conquest over the past four hundred years. They have shown that the number of wars in which territory changes hands as a result of war has markedly decreased over the centuries. Even though territory is still essential to warfare (taking and holding territory is what armies are geared towards), changing one’s political borders through military force has decreased dramatically in the 20th century. Thus the strong reaction to incidents like the 1990 Iraq invasion of Kuwait or more recently the ongoing crisis in Ukraine.

The authors describe the norm against conquest as having its roots in Westphalia, but really beginning to calcify in the 1900s after WWI with the League of Nations (created to mediate disputes between great powers), and initiatives such as the Kellogg-Briant Pact which sought to outlaw wars in general. It should be noted that the reason for legal initiatives like this was not because changing territory through war was not seen as somehow illegitimate, but rather that the experience of WWI had shown the utterly devastating power that modern warfare could inflict on states. The goal was to avoid war in general, not just territorial change through war. Certainly WWII did nothing to disconfirm people’s opinions on the destructiveness of war- and under the International Military Tribunal at Nuremberg Nazi leaders were put on trial for conspiracy to wage “aggressive war” (among many other counts). This focused mainly on the breaking on treaties on Germany’s part, but similar language and ideas were used for the UN Charter. Article 2, paragraph 4 of the UN Charter declares that “... All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the UN” It further states that members of the UN (Article 33) shall seek alternative methods of resolving disputes besides war. Interestingly this does not mean that war to rectify territorial claims can be considered “aggressive war”, but it does make it potentially unlawful.

Now as we all know, wars continue to occur but wars to change one’s territory are much rarer. In fact, most border changes we see today are the result of independence movements or the results of border treaties between claimants than conquerors of other states. Much of the “norm” as it exists today is legal: states as parties to the UN must seek alternative methods to resolve their claims and if they do not then they are in violation of one or more Articles of the UN. This gives other states the basis for claims of aggression to decry territorial acquisitions. The other part of the norm has its basis in morality- changing territory by war is enormously destructive and can be thought of as exceedingly greedy. If claims can be solved through other means, war is not only unlawful but wasteful and morally repugnant.

And there is an underlying assumption that I think you make in the first paragraph which is that “conquest was much more acceptable before modern times- and could even be seen as a good thing (i.e. Ghenghis Khan being considered a great leader)”. I would dispute this, as it’s not terribly difficult to find sources from that time decrying aggression by others or lamenting the depredations of war (Thucydides comes to mind). Those who benefitted from the conquests will of course place accolades on their leader, while those who lost out will vilify the one who started the conflict. Losers typically are not able to create monuments against their conquerors. It would likely not be difficult to find historical Spanish-language newspapers accusing the US of imperialism during the Spanish-American war.

There are also plenty of treaties throughout history designed specifically to prevent wars of territorial acquisition. The Treaty of Tordesillas was intended to prevent wars of conquest between Spain and Portugal over the New World. Even the “big bad” USSR refrained from directly annexing its client states after World War II so as not to be seen as overly aggressive.

This is by no means a complete answer (I may have gone a little off-point in the last few paragraphs), and I welcome other comments/corrections.

Edit: paragraph spacing

There hasn't really ever been a point in history where people were just perfectly fine with conquering. It's more the fact that typically the conquered weren't as vocal afterwards, especially going back to Greek and Roman times where speaking out typically warranted death or at the very least the loss of your wealth, and it would've been easier to just live with what had happened.

Perhaps the best early example that has remained a piece of pop culture to this day is Dante's Divine Comedy, having been defeated and exiled by the Black Guelphs. Dante (A White Guelph), in his writings, (The Inferno primarily) he insulted everyone from the Pope to Black Guelph leaders who he blamed for the misdeeds done against him. After being conquered he had pretty much lost everything and couldn't even return home, and big surprise, he happened to die not too long after his Divine Comedy was printed. Leaving many to this day believing he was murdered by the very people he slandered.

I can't speak on the major current events you highlighted, but when it comes to the Philippines there actually was a significant amount of outcry, especially in the USA who saw it as unnecessary and against our ideals (deja vu), people like Andrew Carnegie, and Mark Twain among others were very vocal against the seizure and war with the Philippines.

Earlier examples of outcry against "conquering" that were especially vocal would be Post-Napoleonic Europe with the Congress of Vienna meeting and redividing up Europe after Napoleon shook up the whole of the continent, leaving Europe in a relatively war-hating state that would persist through the remainder of the 1800's (minus the Crimean War, and some other small issues, but considering the scale of the prior wars it was a good start at peace). Another great example being the world after WW1, with the 20's lifestyle personifying the idea of drinking ones sorrows away after everyone had witnessed firsthand one of the most horrific wars to ever exist. Although it wouldn't last long, certainly the anti-war sentiment was very strong, especially in the USA who would desperately try to avoid getting into WW2 simply because of their hatred of the last war.

So basically although with the internet, the world has become a lot smaller, and in recent years (don't worry not gonna mention current events) it has become a lot easier for the smaller voices to be heard. I think because of that we forget about the people who were silenced, which is understandable, but the voices were definitely there, regardless of whether or not they were heard.

To come at it from an economic or macro-social angle, the key is globalization. Previously, the kingdom of Mali would not have cared or known about Mongol conquests in China or the warring kingdoms of India. France was not disturbed by the presence of Incans or Nobunaga's unification of Japan. There was a notable amount of trade between the Dutch and Japanese but I'd say it's quite little compared to modern times.

Since WWI and WWII, there's been a boom in globalization, especially with communications being easier due to technological changes. Major companies are international. Disney has theme parks all across Europe and Asia and the Americas. Toyota sells and makes cars in multiple countries. Walmart ships an unreal amount of goods from China on ships as large as islands. You want a stable economy, and stable economics partially relies on stable (or exploitable unstable) politics. This guideline is of course applied much more favorably to hegemonic powers like the US. In the case of Saddam taking Kuwait (which is now over the 20 year mark), a big part of it was oil. Kuwait had been overproducing oil and driving down the price of oil to almost $10 a barrel, which was believed to be on the orders of the US. Iraq and other Arab nations suffered heavily from this. There's a lot more to it than this but it serves as an example of how economic decisions are reliant upon a constant political climate, especially with companies that operate outside their own country.