If you polled global security experts today about the parts of the world that keep them up at night, the top three would probably be Ukraine and Crimea, the Korean Peninsula and post-Brexit Western Europe. Coincidentally, the troubles facing all three can be traced back to a short conference held 75 years ago on Tuesday.

The meeting, at the Crimean resort of Yalta, brought together the three top Allied leaders, Franklin Roosevelt, Winston Churchill and Joseph Stalin. The war was winding down, and they were there to determine what came next.

Among the most pressing issues were the borders and future democratic freedoms of Poland, which Roosevelt and Churchill had pledged to safeguard. By February 1945, however, the Red Army was in control of most of Eastern Europe. As Stalin was fond of saying, “Whoever occupies a territory imposes on it his own social system,” and the Soviet Union was simply too powerful to resist. Roosevelt and Churchill battled for self-determination for Poland and other Eastern European countries, but in the end secured only paper promises that the West had no way to enforce and that Stalin violated within weeks.

Poland was the reason Britain had originally gone to war, a fact not lost on Churchill that month. Soon after he returned from Yalta, Churchill even asked his military commanders to assess the cost of using force to compel Stalin to give Poland a fair deal. The response was 45 Anglo-American divisions, several Polish divisions and 100,000 rearmed German troops. The commanders nicknamed the idea of going to war against the Soviet Union “Operation Unthinkable,” which, of course, it was. A year later, Churchill declared that an “iron curtain” had descended across Europe.