In May 1940, AASF and BEF Blenheims participated in the Battle of France, being sent against German forces moving towards Brussels, resulting in many aircraft quickly sustaining heavy damage or being lost to enemy fire. German attacks upon the French airfields also damaged a considerable number of Blenheims on the ground. On 14 May, a combined force of Fairey Battles and Blenheims was dispatched on a counter-attack upon German forces as they broke through defensive lines: 40 out of 71 aircraft were lost in this sortie. This is claimed to be the highest ever losses known to the RAF. Further action by Blenheims of Bomber Command that day sustained a 25% aircraft loss despite a high level of British fighter cover. Shortly thereafter, the mostly-depleted squadrons were withdrawn to Britain. Around 50 Blenheims supported the Dunkirk evacuation by harassing enemy forces.