Complex sentence: The Battle of Arras began on 9 April 1917 and in support the RFC deployed 25 squadrons made up of 365 aircraft, about a third of which were fighters (scouts).
Split into simpler sentences: The Battle of Arras began on 9 April 1917. In support, the RFC deployed 25 squadrons made up of 365 aircraft, about a third of which were fighters (scouts).