Actually not a bad example of what can be done but the USSR was never a nation. Rather, they had invaded other nations and forced them into submission. Had it not been for the wisdom of Gorbachev, it is conceivable the USSR would still exist today or, more probable, another war would have pitted it and western nation. Interestingly, Gorbachev is hated today in Russia and recognized as a great man by the West.
Post colonial UN Charter called for the inviolability of borders as a means to prevent border wars. Yet that foundation has been politicized to mean little and seems to only apply to smaller nations or when it suits western powers. Most don't remember the Biafra war of secession that killed hundreds of thousands in Nigeria but never supported by the West and UN. In a twist, Eritrea seceded from Ethiopia on the basis of established colonial borders and won. Yet, South Sudan won its independence after years of atrocious war between Muslim north and Christian and animist south; a nonsensical decision that contravened the tenets of the UN Charters. The same can be said of the recognition by some western nations of Kosovo as an independent nation, a further demonstration that strict rules really only apply to the weak.
The European ideal sought to do away centuries of border conflicts by removing and placing them further away, where Europe no longer is. Despite many challenges, the effort is working. Europeans can travel, work and live in each others nations, trade in the same currency and ultimately have reduced the risk of war within to an almost impossibility. That was the fundamental goal of this union, something nationalists never mention when whining about their loss of identity.
The US federation has also been tested by war and could be tested again. Despite complicated processes to follow, states could theoretically demand their independence. As a former sovereign nation, Texas could lead the pack. America could also implode.