In the early 1960s U.S. nuclear submarines armed with mid-range nuclear missiles of type Polaris A-1 were deployed outside the Swedish west coast. Range and safety considerations made this a good area from which to launch a retaliatory nuclear strike on Moscow. The submarines had to be very close to the Swedish coast to hit their intended targets though. As a consequence of this, in 1960, the same year that the submarines were first deployed, the U.S. provided Sweden with a military security guarantee. The U.S. promised to provide military force in aid of Sweden in case of Soviet aggression. This guarantee was kept from the Swedish public until 1994, when a Swedish research commission found evidence for it. As part of the military cooperation the U.S. provided much help in the development of the Saab 37 Viggen, as a strong Swedish air force was seen as necessary to keep Soviet anti-submarine aircraft from operating in the missile launch area.