Complex sentence: Douglas Bader was posted to command the Squadron, as a Squadron Leader, at the end of June 1940, that was mainly made up of Canadians that had suffered high losses in the Battle of France and had low morale.
Split into simpler sentences: Douglas Bader was posted to command the Squadron as Squadron Leader at the end of June 1940. It was mainly made up of Canadians who had suffered high losses in the Battle of France and had low morale.