The codename Circus was given to operations of British Royal Air Force (RAF) during World War II where bombers, heavily escorted by fighters, were sent over continental Europe to bring enemy fighters into combat. These were usually formations of 20 to 30 bombers escorted by up to 16 squadrons of escort fighters. Bomber formations of this size could not be ignored by the German Luftwaffe.
At first medium bombers such as the Handley Page Hampden were used but even with heavy escort they were too vulnerable to flak and German fighters. They were replaced with Bristol Blenheim bombers but they fared no better. Heavy bombers such as the Short Stirling were a better fit, but after several missions bomber command needed them back.
The Hurribomber, a fighter-bomber version of the Hawker Hurricane, was used with better results except that the small bomb load caused little damage.
There were other codenames for similar missions.