President Barack Obama’s nuclear diplomacy with Iran may yet fail. On Tuesday, exactly one week before a June 30 deadline for an agreement, Iran’s Supreme Leader delivered his latest in a series of defiant statements, setting conditions for a deal—including immediate relief from sanctions, before Iran has taken steps to limit its nuclear program—that Obama will never accept. Secretary of State John Kerry warned last week that the U.S. is prepared to walk away from the talks. And even if a deal is reached, the story is not over. The Iranians may break or cheat on an agreement, and try build a nuclear weapon anyway.

That’s why, at least three times in the past year, a B-2 stealth bomber has taken off from an Air Force base in Missouri and headed west to the White Sands Missile Range in New Mexico. For these missions, the $2 billion plane was outfitted with one of the world’s largest bombs. It is a cylinder of special high-performance steel, 20 feet long and weighing 15 tons. When dropped from an altitude likely above 20,000 feet, the bomb would have approached supersonic speed before striking a mock target in the desert, smashing through rock and burrowing deep into the ground before its 6,000 pounds of high explosives detonated with devastating force.

“It boggles the mind,” says one former Pentagon official who has watched video of the tests. […]