Original sentence: Before 1917 both countries were ruled by autocrats (the Tsar and the Kaiser), and both could have been attracted to an alliance against the democratic powers of Western Europe (the USA was isolationist, regarding European affairs, until it became a belligerent of World War I in 1917)
Edited sentence: Before 1917 both countries were ruled by autocrats (the Tsar and the Kaiser), and both could have been attracted to an alliance against the democratic powers of Western Europe (the USA was isolationist, regarding European affairs, until it became a participant of World War I in 1917)