Complex sentence: The army at the start of the war remained small in comparison to its enemies and up to 1939, would remain an all volunteer force, by the end of the war over 3.5 million men had served in the army.
Split into simpler sentences: The army at the start of the war remained small in comparison to its enemies and up to 1939 would remain an all volunteer force. By the end of the war over 3.5 million men would have served in the army.