After the German and Soviet invasions, substantial numbers of Polish military personnel escaped to neutral countries, primarily Romania, Hungary, Lithuania and Latvia, where they were interned. Air force personnel mostly evacuated to Romania, from where significant numbers made it to France either overland via Yugoslavia and Italy (still neutral at the time), or by boat; the internment camps weren't very closely guarded, and the Polish government in exile worked hard to assist with transport.

Polish air force units were then formed in both France and Great Britain (around 7,500 personnel in the former, 2,500 in the latter), though neither were particularly enthusiastic about it, the French mostly assigning obsolescent aircraft, the RAF forming two bomber squadrons flying Fairey Battles. Following the fall of France, Polish units were evacuated to the UK from western French ports in Operation Aerial, allowing the RAF to form a further two bomber squadrons and two fighter squadrons.