Additionally, the US has now been engaged in Afghanistan longer than they were in Vietnam. There's a whole generation of Americans who know all about war. In the US, the generation who served in Afghanistan and Iraq, then lived through the Great Recession are about to go into politics.

My prediction is that if we ain't seen nuthin' yet.

The decline of agriculture as a proportion of the economy of advanced nations means that territory no longer has the value that it once did. Population is not as important as it used to be for military power. Physical capital (factories, labour, land) is now less valuable, certainly in the developed world, than human capital (education, skills, networks). This means the gains of conquest have reduced drastically.

Meanwhile, the costs of war have increased markedly. Only a small group of states can afford to build technologically competitive militaries. Nuclear war has made the cost of great power war mutual annihalation. Modern low intensity warfare, on the other hand, makes it extremely difficult for great powers to permanently occupy even small powers (e.g. the US in Vietnam, USSR in Afghanistan). This means that the costs far outweigh the benefits in almost all circumstances. Add to this that in the counterfactual, states can gain most of the economic benefits of war through peaceful means, i.e. trade, at a far lower cost. There's also simply a greater spread of uncertainty in war: it's a dictum of war studies that war is inherently unpredictable.

If international war breaks out in the near future, it will be a fight over global hegemony (US-China), regional hegemony (Iran-Saudi Arabia), revanchism (Russia, Taiwan), or most likely - by accident.