Complex sentence: Hitler had successfully taken control of Austria and Czechoslovakia by early 1939, when Britain and France reversed their policy of appeasement and switched to a policy of deterrence, warning they would declare war if Germany attacked Poland.
Split into simpler sentences: Hitler had successfully taken control of Austria and Czechoslovakia by early 1939. Then Britain and France reversed their policy of appeasement and switched to a policy of deterrence, warning they would declare war if Germany attacked Poland.