Gen. David Goldfein

(Source: Marcus Weisgerber/DefenseOne) BARKSDALE AIR FORCE BASE, La. — The U.S. Air Force is preparing to put nuclear-armed bombers back on 24-hour ready alert, a status not seen since the Cold War ended in 1991. That means the long-dormant concrete pads at the ends of this base’s 11,000-foot runway — dubbed the “Christmas tree” for their…