I do not know the value of life. In every war zone that I find myself in, I routinely fail to establish a sensible line beyond which I will not take risks, just as I struggle to pass judgment on war itself. When is killing justified? When is risking my life to report on killing justified? Most of what I have seen is unacceptable, but some of it is not—a nation defending itself against genocide, or a nation liberating itself from tyranny. The parameters of war are liquid, like blood.

In the days leading up to the invasion of Iraq, I saw a well-traveled friend writing in his diary. He had been keeping it for nine years, writing in it every day. "When I look back on the early passages," he told me, "I realize that I have learned nothing." I don't keep a diary, but his words made me wonder how much—and what—I have learned about war. I was asking these questions as I drove into Iraq on the first day of the invasion, and by the time I arrived in Baghdad, three weeks later, I had found some answers.

IT WAS SUPPOSED TO BE A WALKOVER. American troops would cross into Iraq and meet surrendering soldiers and grateful civilians, and I would write a story about the happy liberation of the southern city of Basra.

The invasion began on the night of March 19. I'd been staying with some other journalists in a house in the northern Kuwaiti desert, a few miles from the Iraqi border. Packing up that night, we could hear everything—the 2,000-pound smart bombs landing on Iraqi trenches and the waves of Apache gunships flying 50 feet overhead, eerily invisible, moving without lights.

I was on assignment for The New York Times Magazine with French photographer Laurent Van der Stockt. We made our last preparations for the 40-mile drive to Basra, bolting luggage racks onto our two rented SUVs and strapping down jerry cans filled with gasoline. We figured it would be a short trip, but in case light skirmishing delayed the city's liberation, we packed a few other prudent items: sleeping bags, cans of tuna, chocolate bars, gallons of drinking water, body armor, Kevlar helmets, biochemical suits, U.S. military uniforms, two spare tires, a stove, satellite phones, shortwave radios, and, in Laurent's Mitsubishi Pajero, a box of Cuban cigars.

We headed out at 4 a.m., hoping the thousands of fighting vehicles storming across the desert would create enough havoc to let us slip across the border. Laurent led the way, because his vehicle had an onboard compass and GPS system; at the wheel of my Hyundai, I followed him along one back road and then another, neither of us sure where they led, except toward the war. Shortly before dawn, we passed through a gap in the line of sand berms the Kuwaitis had created on their side of the demilitarized zone; I could see flashes of artillery and tank fire a mile or two ahead. We were close. Then I heard the shouting. "Turn off your fucking lights! Turn them off now!"

We stopped and turned off our lights. The American soldiers who appeared out of the darkness had a Special Forces look, with black caps and assault rifles outfitted with high-tech accessories. They were not happy to see us.

"We almost lit you up," one of them said. "What the fuck are you doing here?" They ordered us to turn around.