Difficult to do in a post WW2 world where the mantra is "borders are sacred, don't disturb them." Modern international law holds territorial integrity and self determination as separate but equal concepts that should always be maintained no matter what. Situations where those two concepts directly oppose each other can get messy fast.
When Ukraine seceded from the USSR they took Eastern Ukraine and the Crimea with them when they became sovereign. You do not want Ukraine to become sovereign in TTL, at least not right away. Have the USSR be replaced by a new union and keep the Eastern European and the Central Asian states in it. As it becomes clear that this new union will be Russian dominated this will cause a crisis in Ukraine. The Ukrainian Nationalists will want independence from the Union and the Ethnic Russians will want to stay in it.
This situation give Russia a Casus beli to swoop in and breakup Ukraine in support the self determination of the Russian population there. To the eyes of the world, since Ukraine is not sovereign, this will be treated like a civil war and not an invasion of an internationally recognized country. It might cause a stink, like the Chechen Wars caused in OTL but there will be no chance of intervention or sanctions here. The remainder of West Ukraine will probably declare independence and might end up joining NATO and the EU eventually but they will have to renounce the East under the "no territorial disputes clause" so this should not trigger a war.