From the 1950s' Pentagon to today's Obama administration, the United States has repeatedly pondered, planned and threatened use of nuclear weapons against North Korea, according to declassified and other US government documents released in this 60th-anniversary year of the Korean War. Air force bombers flew nuclear rehearsal runs over North Korea's capital during the war. The US military services later vied for the lead role in any "atomic delivery" over North Korea. In the late 1960s, nuclear-armed US warplanes stood by in South Korea on 15-minute alert to strike the north. Just this April, issuing a US Nuclear Posture Review, defense secretary Robert Gates said "all options are on the table" for dealing with Pyongyang -- meaning US nuclear strikes were not ruled out. The stream of new revelations about US nuclear planning further fills in a picture of what North Korea calls "the increasing nuclear threat of the US," which it cites as the reason it developed its own atom-bomb program -- as a deterrent. "This is the lesson we have drawn," North Korea's vice foreign minister, Pak Kil Yon, told the UN in September. Expert observers are speculating that North Korea, which conducted nuclear tests in 2006 and 2009, may soon stage another. But US threats go back more than a half-century, to long before North Korea split its first atom.