He was a leader of the Civil Rights Movement, the American labor movement, and socialist political parties. He<span> organized and led the Brotherhood of Sleeping Car Porters, the first predominantly African-American labor union.</span>
Decisive or critical, especially in the success or failure of something/ of great importance
I believe it's A: capitalizing on the harsh post-war conditions in their countries.
The Soviet Union - they were the cause and driving force behind Communism in Eastern Europe from the 1920/30s into the 80s. Without their military power and influence, communism in Eastern Europe fell away.
