Why did the “big three” meet yalta in january 1945?

The Big three (Great Britain, the Soviet Union, and the United States) meet in Yelta in 1945 to discuss the end of World War II along with what the world will look like after the war. Winston Churchill (Prime Minister of Great Britain), Joseph Stalin (leader of Soviet Union), and Franklin D. Roosevelt (President of the US) agreed that Germany must be forced to surrender. Along with this, they talked about how Europe will look after the war. This included territories/countries in Eastern Europe having the ability to elect their new leaders.

The answer is ethnic groups.