The British hydrogen bomb programme demonstrated Britain's ability to produce thermonuclear weapons in the Operation Grapple nuclear tests in the Pacific, and led to the amendment of the McMahon Act. Since the 1958 US–UK Mutual Defence Agreement, the US and the UK have cooperated extensively on nuclear security matters. The nuclear Special Relationship between the two countries has involved the exchange of classified scientific data and fissile materials such as uranium-235 and plutonium. The UK has not had a programme to develop an independent delivery system since the cancellation of the Blue Streak in 1960. Instead, it has purchased US delivery systems for UK use, fitting them with warheads designed and manufactured by the UK's Atomic Weapons Establishment (AWE) and its predecessor. Under the 1963 Polaris Sales Agreement, the US supplied the UK with Polaris missiles and nuclear submarine technology. The US also supplied the RAF and British Army of the Rhine with nuclear weapons under Project E until 1992. Nuclear-capable American aircraft have been based in the UK since 1949, but the last US nuclear weapons were withdrawn in 2006.