Complex sentence: The Eagle Squadrons were fighter squadrons of the Royal Air Force formed during World War II with volunteer pilots from the United States, similar to the model set by the Polish 303 Squadron (see Battle of Britain Foreign Contributions).
Split into simpler sentences: The Eagle Squadrons were fighter squadrons formed during World War II with volunteer pilots from foreign countries. Initially they were Royal Air Force fighter squadrons made up of pilots from the United States of America, similar to the model set by the Polish 303 Squadron (see Battle of Britain Foreign Contributions).