A sprawling new plant [near Kansas City] in a former soybean field makes the mechanical guts of America's atomic warheads. Bigger than the Pentagon, full of futuristic gear and thousands of workers, the plant, dedicated last month, modernizes the aging weapons that the United States can fire from missiles, bombers and submarines. It is part of a nationwide wave of atomic revitalization that includes plans for a new generation of weapon carriers. A recent federal study put the collective price tag, over the next three decades, at up to a trillion dollars. This expansion comes under a president who campaigned for a nuclear-free world" and made disarmament a main goal of American defense policy. Supporters of arms control, as well as some of President Obama's closest advisers, say their hopes for the president's vision have turned to baffled disappointment as the modernization of nuclear capabilities has become an end unto itself. "A lot of it is hard to explain," said Sam Nunn, the former senator whose writings on nuclear disarmament deeply influenced Mr. Obama.