The world hasn't had a major war between great peer powers since WWII. Everyone who was old enough to remember that war is now dead, or very close to it. There's a few surviving veterans, I think, but they're almost dead of old age, and they were just teenagers during the war.

So, in short, humanity has collectively forgotten the horrors of such a war.

Also, similar to WWII, we have a large country full of people who think they're somehow entitled to conquering all their neighbors, establishing a new empire, and being a great world power under their dear leader, but this time they have nuclear weapons.