X-51 Hypersonic Cruise Missile: The Pentagon's Prompt Global Strike Weapon Plan - Popular Mechanics:
First, there's the matter of intelligence. If a president is going to launch the first intercontinental ballistic missile attack in history, he'll need overwhelming evidence. Our ability to nail down that kind of quality information is patchy, at best. On March 19, 2003, the United States launched 40 cruise missiles at three locations outside Baghdad in hopes of killing Saddam Hussein and other senior military officials. It turned out the former Iraqi leader wasn't in any of the locations; the strikes killed at least a dozen people, although it's not clear if they were civilians or leadership targets.
The mission failed even though friendly forces controlled the area. At the heart of Prompt Global Strike is a much darker scenario: American troops are far from their intended target — or the enemy's air defenses are too tough to penetrate. "So let me get this straight," says Jeffrey Lewis, a Harvard University nuclear energy and weapons analyst. "We've got exquisite, fleeting intelligence in an area of immediate concern, but no forces nearby and, miraculously, a sub in just the right spot to attack. I suppose there's some chance of that. But it's pretty small."
More difficult to explain is how a conventional Trident could be launched without provoking a crisis even bigger than the one that it was meant to solve. The Navy's plan calls for arming Ohio class subs with two conventional and 22 nuclear Trident II missiles. (The Navy intends to cut its Ohio class fleet from 18 to 14 subs, with 12 in the water at any one time.) To outside observers, the subs' conventional and nuclear weapons would appear identical — the same size, the same speed, shooting from the same location.