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.
Clearly then, you do not believe that repeated and organized violations of a country's borders is not a test of that country's sovereignty. You are on your own there.
I just went at length to describe the EU elimination of its countries internal borders. At the time, nationalists from the wealthier northern nations screamed that hordes of workers from the poorer southern nations would invade them. Years later, we find that fear was totally misguided as wealthier people from the northern countries vacationed and retired in the poorer Mediterranean countries in far more numbers than workers from the south moving north for jobs. America isn't Europe but if instead of propping up dictators in Central America to protect our corporations and selling weapons to local gangs we helped create prosperous nations, we wouldn't see so many knocking at our door. Do you see any Costa Rican begging to be let in? Do you know how many Americans live their retirement in Mexico (1.6 million)?