Was it a gradual change?

It was essentially a gradual change. The Peace of Westphalia at the end of the Thirty Years War helped to solidify the concept of Sovereignty, that the State is an entity in with certain rights (/u/elos_ explains it better [here] (https://www.reddit.com/r/AskHistorians/comments/3kz84r/why_didnt_belgium_just_allow_military_access_to/cv1unhc) ), and all states (European at least) had this right. Following the Napoleonic Wars, Swiss neutrality was guaranteed by the Great Powers represented at Vienna, and the Treaty of London recognized and guaranteed Belgian neutrality; Belgium and Switzerland would defend their neutrality if attacked, and in Belgium's case all the Great Powers promised to guarantee this, and in 1914, Britain did.

In the Imperial 'world', the Congress of Berlin was arranged by Bismarck, precisely in order to divide up Africa so as to avoid conflicts over territories there. This helped lend some 'method to then madness' of the Scramble for Africa, which was essentially over by the Moroccan Crises of 1905 and 1912, that ended with French and Spanish control of Morocco. There are other examples such as the Anglo-Russian Arrangement of 1907, that divided up spheres of influence in Persia/Iran, and the spheres of influence created in China, to avoid major confrontation between the Great Powers, which aside from the Russo-Japanese War, was achieved.

There were also powerful arguments mooted against the idea of wars of conquest themselves, the most notable of which, perhaps, was Norman Angell's The Great Illusion, which essentially put forward a modernized notion of Capitalism making war obsolete. Angell suggested, and he was in line with Ivan Bloch, that because modern capital relied on industries, trade, and the societies that created and sustained them, modern war was impossible, or if attempted, suicidal, as it would fundamentally disrupt the economic balance, ruin or damage economies, and presumably prompt revolutions that would destroy the societies that had gone to war. As Richard Hamilton shows [here] (https://www.youtube.com/watch?v=4e_RfFzyris), Angell's arguments had a considerable following; he would oppose WWI and ultimately win the Nobel Peace Prize afterwards. There were also the writings of those like German General Colmar Freiherr von der Goltz, who argued that modern wars would be long, drawn out volks krieg or People's Wars, tying in here the difficulties that the ideas of nationalism created for a would-be conqueror; these wars would involve the whole of societies, and would be long, bloody struggles.

World War One took place when international law was emerging as a major issue in international relations; Isabel Hull has an excellent lecture about that [here] (https://www.youtube.com/watch?v=CW4j-jjS5ow), that goes into further detail. The Invasion of Belgium became a cause celebre for the Allies, as the Germans violated Belgian neutrality, denouncing one of the oldest and strongest treaties in modern Europe as 'a scrap of paper'. The war ended with the creation of the League of Nations, with the introduction of LoN Mandates that set a limit on Franco-British presence in former German colonies and Ottoman territories, and introduced the concept of National Self-Determination, undercutting the justifications of Imperialism. The Leipzig War Trials never panned out, but would have prosecuted the Kaiser for waging aggressive war, a crime that ultimately would be pinned to German and Japanese leaders at Nuremburg and Tokyo in 1945-46.

Has anything changed or is it a history written by the victors kind of thing?

To quote /u/Tiako:

"History is not written by the victors. I sympathize with people saying that because it is an easily understandable way to convey the problematic nature of written evidence, but it has the effect of replacing one misleading, monolithic interpretation of history ("our sources are trustworthy") with another misleading, monolithic interpretation of history ("our sources are untrustworthy"). "One size fits all" models of historiography are never applicable for individual historians, who are after all complex and loaded with their own personal biases.

Take Tacitus' Agricola as an example. It would be hard to argue against the fact that the Romans "won" in Britain by the time he was writing, given that the titular governor had led his soldiers all the way up to the north of Scotland (this has been confirmed by archaeology, by the way). But Tacitus' sympathies are clearly with the British who he sees as fighting for their liberty against the morally degenerate tyranny of Rome. If you think that winners write the history books, you will think that, ok, the Romans won, and Tacitus is a Roman, he should be biased against the British so anything he says favorable to them can be take as accurate. Many writers have indeed taken this position, and it is absurd.

If you want one nice, "one size fits all" historiographic model that is a good starting point to examine the individual writer's bias, it is that history is not written by winners, but by writers. For most of history the writers came from a distinct social class, not necessarily at the economic and political top, but far from the bottom. They were also highly educated and thus concerned with the preoccupations of the highly educated in that particular society.

To give another illustrative example of this, the Chinese literati were very much against the Mongols despite the Mongols being some of the more impressive victors in all of history. This is because the administrative policies of the Yuan dynasty were often unfavorable to the position of the literate class, who were the administrative backbone of the preceding dynasty.

But even that is no substitution for specific and nuanced examination of the individual writers themselves. The biases of Tacitus are not those of Suetonius."

In fact, this issue fits nicely with WWI, International Law, and that lecture by Hull. The Allies included Article 231 in the Treaty of Versailles to establish a legal framework for German and Central Powers reparations, by asserting their responsibility for the damages caused to the allies (in this case Britain, Belgium, France, USA primarily) by their aggression. International Law had been an important point of contention for the Allies in WWI, and things like the Geneva Conventions, the Washington Treaty, and the League of Nations, were part of their efforts after the war to strengthen it. The Germans, in a remarkable case of when the 'losers wrote history' (as I explain [here] (https://www.reddit.com/r/AskHistorians/comments/34nwwx/after_the_armistice_and_treaty_of_versailles_in/) ), undercut the treaty by turning the key issue from one of reparations for German aggression, to 'who started the war'. Holger Herwig has written an excellent article about it, Clio Deceived (vi.uh.edu/pages/buzzmat/DH%20articles/HerwigClioDeceived.pdf), and Hull points out in her book A Scrap of Paper that German efforts not only succeeded in undercutting the Treaty, but in trivialising the reasons for which the Allies fought the war in the first place.

Other users may add to this, or have different ideas, but I hope I've at least managed to highlight some key 'milestones'.