Initially after the end of World War II, Sweden quietly pursued an aggressive independent nuclear weapons program involving plutonium production and nuclear secrets acquisition from all nuclear powers, until the 1960s, when it was abandoned as cost-prohibitive. During the Cold War Sweden appeared to maintain a dual approach to thermonuclear weapons. Publicly, the strict neutrality policy was forcefully maintained, but unofficially strong ties were purportedly kept with the U.S. Sweden, for instance, cooperated extensively with U.S. intelligence: "Though officially neutral, Sweden in fact built very close ties to both <abbr>NATO</abbr> and the US security establishment in the late 1940s and early 1950s and was deeply involved in cold war spying operations."