hydrogen bomb, and a favourable international relations climate created by the Sputnik crisis, led to the McMahon Act being amended in 1958, and the restoration of the nuclear Special Relationship in the form of the 1958 US–UK Mutual Defence Agreement (MDA), which allowed Britain to acquire nuclear weapons systems from the United States. Britain's nuclear weapons armament was initially based on free-fall bombs delivered by the V bombers of the Royal Air Force (RAF), but the possibility of the manned bomber becoming obsolete by the late 1960s due to improvements in anti-aircraft defences was foreseen. In 1953, work began on a medium-range ballistic missile (MRBM) called Blue Streak, but by 1958, there were concerns about the vulnerability of this liquid-propellant-missile to a pre-emptive nuclear strike. To extend the effectiveness and operational life of the V bombers, an air-launched, rocket-propelled standoff missile called Blue Steel was developed, but it was anticipated that the air defences of the Soviet Union would improve to the extent that V bombers might still find it difficult to attack their targets. A solution appeared to be the American Skybolt missile, which combined the range of Blue Streak with the mobile basing of the Blue Steel, and was small enough that two could be carried on an Avro Vulcan bomber. An institutional challenge to Skybolt came from the United States Navy, which was developing a submarine-launched ballistic missile (SLBM), the UGM-27 Polaris. The US Chief of Naval Operations, Admiral Arleigh Burke, kept the First Sea Lord, Lord Mountbatten, apprised of its development. By moving the deterrent out to sea, Polaris offered the prospect of a deterrent that was invulnerable to a first strike, and reduced the risk of a nuclear strike on the British Isles. The British Nuclear Deterrent Study Group (BNDSG) produced a study that argued that SLBM technology was as yet unproven, that Polaris would be expensive, and that given the time it would take to build the boats, it could not be deployed before the early 1970s. The Cabinet Defence Committee therefore approved the acquisition of Skybolt in February 1960. The Prime Minister, Harold Macmillan, met with the President, Dwight D. Eisenhower, in March 1960, and secured permission to buy Skybolt. In return, the Americans could base the US Navy's Polaris ballistic missile submarines in the Holy Loch in Scotland. The financial arrangement was particularly favourable to Britain, as the US was charging only