Complex sentence: The Outer Mongolian revolution of 1921 was a military and political event by which Mongolian revolutionaries, with the assistance of the Soviet Red Army, expelled Russian White Guards from the country, and founded the Mongolian People's Republic in 1924.
Split into simpler sentences: The Outer Mongolian revolution of 1921 was both a military and political event. With the assistance of the Soviet Red Army, Mongolian revolutionaries expelled Russian White Guards from the country, and created a new state, named the Mongolian People's Republic in 1924.