I do honestly suspect this is about delusions of historical grandeur. Ukraine is after all the historical home of Kievan' Rus (with whom most peoples in the region claim historical antecedence). In its "golden age" Vladimir the Great (Volodymir I, Prince of Novgorod) united the Rus peoples under his reign.
Not "medically insane", I'm talking like "Hitler insane" (who IIRC, was never actually proven to be crazy or with an actual medical disability). Attacking peaceful nations for no reason is warmongering and clearly evil... but doubly so when they're effectively brother countries.
Weapons were too good in the early 1900s, and the wear-and-tear on soldiers would scar them for the rest of their lives. The Axis powers didn't get the message in the 1930s/1940s, so one more ass-kicking was needed, but its overall not a good idea to initiate wars of conquest in today's society.
It’s why China doesn’t invade Pakistan or India, aside from small skirmishes that are in borderlands that are effectively proxy wars.