Sometimes, friendship isn’t forever.
At the conclusion of World War II, the old structures of power were a shambles. The traditional European powers were greatly weakened by years of total war and widespread destruction.
The USSR was looking to expand its sphere of influence in Eastern Europe, and at the same time, the United States was assisting with the rebuilding of Western Europe (with some hegemonic strings attached).
As two nuclear-armed superpowers emerged, the world entered the Cold War.