deterrent aimed at Soviet cities was so important that, when it offered to place the V Bombers under SACEUR authority in 1953 in exchange for American financial aid to purchase new fighters, it refused to agree to them being used in a tactical role against Soviet targets in Europe. The agreement permitted Britain to commit only nominal forces to SACEUR, and presaged future technology and targeting cooperation. As the USAF began in 1955 helping the RAF to convert V bombers to carry American atomic weapons under Project E and hydrogen weapons under Project X, cooperation increased and the United States began sharing some war plan details. Although both nations remained reluctant to fully share their plans—as late as 1956, Britain did not have targeting information even for SAC aircraft it hosted—redundancies were eliminated by one side asking the other whether it planned to attack various targets. In February 1959, the USAF agreed to target 150 Soviet bases that threatened Britain with nuclear weapons, while V bombers would use nuclear weapons to attack Soviet air defenses before SAC arrived. The RAF retained a separate plan to attack 30 Soviet cities with hydrogen bombs. The agreement formed the basis for the ongoing nuclear-targeting cooperation between the two countries, and the different target types resembled the two nations' different priorities during the Combined Bomber Offensive of World War II. The Anglo-American dispute during the 1956 Suez Crisis only briefly disrupted the partnership, and the desire to restore relations to their former level, and the Sputnik crisis, increased American willingness to help Britain improve its atomic weaponry. In March 1957 the United States agreed to sell 60 Thor IRBMs, in 1958 American hydrogen-weapon designs, in 1960 the Skybolt ALBM, and after its cancellation the Polaris SLBM in 1962 as replacement. Polaris was especially notable; British officials initially refused to believe the Americans' offer of state-of-the-art submarine missiles at a moderate price, and one scholar later called it "amazing". While its contribution to SIOP was minor compared to the enormous SAC arsenal of 1,600 bombers and 800 missiles, as RAF officers who worked with the Americans rose to leadership positions their experience benefited later partnerships between the two countries. The joint targeting plan changed over time; the 1962 list for the RAF included 48 cities, six air-defense sites, and three bomber bases, and the 1963 list had 16 cities, 44 airfields and other offensive sites,