The United States has about 180 B61 gravity nuclear bombs based in Europe. They are the detritus of the cold war, tactical weapons deployed in Belgium, Germany, Italy, the Netherlands and Turkey to protect NATO allies from the once-feared Soviet advantage in conventional arms. But the cold war is long over, and no American military commander can conceive of their ever being used. Even so, President Obama has put $537 million in his 2014 budget proposal to upgrade these bombs. When all is said and done, experts say, the cost of the rebuilding program is expected to total around $10 billion — $4 billion more than an earlier projection — and yield an estimated 400 weapons, fitted with new guided tail kits so that they are more reliable and accurate than the current ones.