At midday, the Baghdad marketplace was teeming with humanity. Mothers with children in tow were doing their shopping.

That is exactly what the would-be martyr -- bomb strapped to his chest -- was hoping to see, when the vehicle bringing him to the market was stopped two blocks short of that destination at a U.S. checkpoint.

The bomber burst from the car, running toward the market. A U.S. solder shouted in Arabic: "Stop!" The bomber didn't. With adrenalin pumping, heart racing, nerves twitching, another soldier took aim as well as he could. There was no time to think twice. His purpose was not to kill the bomber. It was to save the innocent. He had never killed a person before and never would again. But now, he squeezed the trigger. A bullet ripped through the bomber's neck -- and an aspiring mass murderer went to meet his Maker one block short of his intended victims.

So, did this hypothetical soldier commit an immoral act? Was this a murder the deliberate taking of an innocent life? Or was it an unavoidable and legitimate act in defense of innocent life?

Let me give you a second case.

It's the wee hours of a Sunday morning in some future February. An al-Qaida cell has hidden a bomb inside the stadium where tens of thousands will gather that day for the Super Bowl. A caller in Pakistan dials a number in the United States. Voices on both ends greet one another in Arabic, not the native tongue of Pakistan. A U.S. spy satellite intercepts the call; an NSA computer records it. The computer has no warrant and no probable cause to believe this call will produce evidence of a crime. It is just programmed to record certain types of calls. This is one of them.

"Is Operation Hail Mary set?" says the voice in Pakistan.

"It will end with a bomb in the fourth quarter," responds the voice in the United States.

"Be careful," says the voice in Pakistan. "The Americans have ways of tracing these calls."

"The package is too cleverly hidden," says the voice in the states. "This Super Bowl will be a victory for the faith."

"Insha'allah," says the voice in Pakistan.

Fourteen hours later the stadium is filling with fans as an analyst translates the call. Precious minutes later, the director of national intelligence is heading to the White House to brief the president. He arrives shortly before kickoff.

Meanwhile, federal agents arrest the U.S. citizen whose voice was on the American side of the intercepted call.

"Where is the bomb?" an agent asks him. Continued...