Although after World War II the formal military alliance between the United States and United Kingdom no longer existed, American postwar war plans required using British air bases until the United States developed ICBMs and long-range bombers. American General Carl Spaatz and Chief of the Air Staff Lord Tedder informally agreed in 1946 to US aircraft using British bases. The discussions, and the subsequent actions such as extending runways, were so secret that it is unclear whether Prime Minister Clement Attlee was aware of them. By 1948, the year of the Berlin Blockade, British leaders expected that "in a future world conflict, US and British forces will find themselves fighting side by side" although the alliance had not been formally renewed. The two countries began coordinating their plans for a Soviet attack in Europe after the Czechoslovak coup d'état of 1948, and later that year General Curtis LeMay, head of Strategic Air Command (SAC), asked Tedder to allow the basing of American atomic weapons in Britain. By the end of 1948, several British bases were atomic-capable or were close to being so, but the ability to fight an atomic war from Britain did not exist until April 1949 when Silverplate B-29 bombers began rotating through the bases, and no American atomic weapons were present in Britain until 1952.