Complex sentence: When Poland was invaded by the German Army in 1939, he escaped to London, where he joined Wladyslaw Sikorski and Stanislaw Mikolajczyk to establish a Polish Government in Exile.
Split into simpler sentences: When Poland was invaded by the German Army in 1939. He escaped to London were he joined Wladyslaw Sikorski and Stanislaw Mikolajczyk to establish a Polish Government in Exile.