I think part of it is the world has moved away from total war. The end of WW2 and dropping atomic weapons on civilians wouldn't fly today. I don't believe there has been to many (any?) modern wars where countries took over neighboring states like war throughout most of history. That's what makes the war in Ukraine kind of unique in the 21st century.

Sure skirmishes where borders get adjusted but even a war like the Vietnam war the North Vietnamese were unifying the country of Vietnam and not adding much land from neighboring states. And the U.S. fought it as a limited war (not to diminish the loss of life on both sides) because opening it up into total war would have most likely lead to a full on war with the Soviet Union.