The Second World War created the largest refugee crisis in history.

I can't speak for every single situation, but one refugee crisis that I do know about was with regards to Germans in the East. This group included people from what was then Germany proper and is now Poland and Russia (now Kaliningrad Oblast) as well as Germans from Czechoslovakia (known as Sudetendeutsche) and Southeastern Europe (primarily Hungary, Romania and Serbia) who were known as Donauschwaben or Danube Swabians and to a lesser extent Siebenbürger Sachsen or Transylvanian Saxons. These people were the descendants of Germans invited to settle areas that had been liberated from the Ottoman Empire and emptied by Turks and other Muslims fleeing toward Anatolia.

As the Soviet Union advanced eastwards, entire cities full of Germans fled west as they heard rumors of mass rapes and murders by the Red Army. Many who didn't flee were expelled east of the Oder-Neisse line which is the current border between Germany and Poland.

As for the Donauschwaben and Siebenbürger Sachsen, the majority of them were forcibly expelled by the newly set up communist regimes in the countries that they resided in.

These people mainly went to Germany, specifically the sectors of British, American and French occupation that would become West Germany and numbered around 16 million people.