Much of the international relations literature is concerned with exactly this question, why do states go to war; and more recently why do they not?

With a social constructivist interpretation in mind, that is, how norms shape identity and then how these identities are filtered upward to the international theatre, there has been a change in how states perceive war. According to scholars like Mueller, World War I marked the beginning of the end for much of European war. This was because previously the act of war was significantly less lethal; think for example the occurance of trench warfare and machine guns, an extremely deadly mix. Also, the beginning of the use of chemical weapons, though not necessarily as devastating, established technology in war as a negative, and thus war itself as a negative.

According to many of these same scholars, who in truth may skimp on the history a touch in favour of IR theory, the League of Nations was an attempt to limit warfare and it's biggest opponents were those who had not undergone some sort of normative change. Notably, the Nazis saw the potential for glory in war whereas other states, like the UK, France and the US, did not necessarily, to the extent that it wasn't cemented within their ideology. Therefore we can see here divergence in attitudes in war between a set of states that would only be fully cemented into Europe after WWII.

These are the views that I largely subscribe to and write about but there are others too, some normative and others material that I'll try to get across here too.

There is another theory widely known within IR known as the democratic peace theory that is at times controversial and has been accused of cherry picking cases. This theory is vast and I can't possibly cover it all here, but the idea is that democracies do not go to war with one another, whereas non-democracies will go to war with others states at a higher rate. The reasons for this can be normative, I.e., democrats do not see themselves as warmongers; they could be mechanical in many ways, politicians know they cannot wage wars without risking their popularity, lastly, it may be that democracy encourages openness and transparency and thus a war between two democracies is unlikely because each of them could see it coming and being debated within their democratic forums. There really is a lot to this theory and I strongly encourage you to explore it further.

What I have tried to do here is emphasize the role of citizens in determining the course of war and how it is waged. There is of course much more to be said. However, on your question of where war can be said to have changed from desirable to undesirable, some point to the end of WWI and then WWII definitively within Europe and others suggest that that is occurred with the growth of democracies in the world, which has grown increasingly since WWI. Therefore, the IR literature offers some indications of how war has fallen out of favour, but no theory reigns supreme.

Sources

Fukuyama, F., 1989, The End of History, The National Interest

Jervis, R., 1992, The Future of World Politics, Will it Resemble the Past?, International Security, Vol 16, No 3

Mueller, J., 1989, Retreat from Doomsday: The Obsolescence of Major War, Basic Books

Mueller, J., 2009, War Has Almost Ceased to Exist", Political Science Quarterly, Vol 124, No 2

Oren, I., 1995, The Subjecticity of Democratic Peace: Changing US Perceptions of Imperial Germany, International Security, Vol 20, No 2