Russia wants to create a buffer zone. If Russia is forced to take Ukraine back, then they will have to create another buffer zone. Theoretically, this could continue until Russia takes all of Europe. This isn't going to happen, but theoretically it could.
Ah! I read "Who Will Tell The People" by William Greider in the late 80's. It was a book my mother had to read in college. As a child, I read "Guadalcanal Diary" and first-hand accounts of the Vietnam War, because my parents would just drop me off at the library while they ran errands. I soon realized that "war is a racket."