Many countries have border problems. For some, the problem is keeping people out. Others have difficulty keeping people in. While I believe in liberal immigration policies, I'm glad that I live in one of the keeping-people-out countries.
Marxist-Leninist regimes have always had the opposite problem. Amid all their talk about the happiness of the New Soviet Man (in a more sexist era) and great increases in tractor production under the latest five-year plan, their emigration policies always belied their propaganda.
During 1989, on the border between Austria and Hungary, two formerly united countries that, after World War II, found themselves on opposite sides of the Iron Curtain, the system of emigration control broke down. I consider this to be one of the most significant events of that eventful year, the importance of which is underappreciated.
On May 2, Hungary began dismantling the fences on its border with Austria. That made it relatively easy for people to cross the border illegally. The impact of that move went beyond the issue of keeping Hungarians at home.
Communist governments had allowed their people relatively free movement between Soviet bloc countries. While they all maintained the same border controls, there was relatively little danger of their people using that opportunity to defect.
East Germans, who had traveled to Hungary, began to cross into Austria, from which they could easily enter West Germany. That increased the pressure on East Germany and similar countries, to either crack down on liberalization (as the Chinese regime did, around that time), or give up and become free-market democracies.