Complex sentence: At the beginning of the Second World War in 1939 the British Army was around 200,000 strong, but with the introduction of the National Service and the call up of Army reservists the Army increased to 2,000,000 men.
Split into simpler sentences: At the beginning of the Second World War in 1939 the British Army numbered around 200,000. With the introduction of the National Service and the activation of reservists, the Army increased to 2,000,000 men.