Complex sentence: Churchill's 69th birthday occurred during the latter conference, and she attended his birthday party which was held at the British Embassy in Tehran, which was also attended by U.S. President Franklin D. Roosevelt and Soviet Marshal Josef Stalin.
Split into simpler sentences: Churchill's 69th birthday occurred during the latter conference, and she attended his birthday party which was held at the British Embassy in Tehran. It was also attended by U.S. President Franklin D. Roosevelt and Soviet Marshal Josef Stalin.