Drawing on the experience of Operation Avalanche, Eisenhower noted that it was "vital that the entire sum of our assault power, including the two Strategic Air Forces, be available for use during the critical stages of the attack." The final landing plan, largely devised by Montgomery, called for nine divisions to land at five zones. Three divisions of airborne infantry would land behind the enemy lines in order to seize vital roads and bridges. Originally scheduled for May 1944, Operation Overlord was pushed back a month, largely due to landing craft shortages. For the same reason, Operation Dragoon, the Allied landing in Southern France, would take place in August 1944 rather than contemporaneously with Overlord, as had originally been planned. Eisenhower insisted that the British give him exclusive command over all strategic air forces to facilitate the landing, to the point of threatening to resign unless Churchill relented, which he did.