Our company commander stressed that we should exact only as much harm as the mission required, but a tank is not a scalpel. When we drove across a field, newly planted crops flew skyward behind our vehicles in great roostertails of earth. When we provided supporting fire to an engineer detonating mines, we felled trees with our machine guns. Inside the tank, we hardly felt a bump when we crushed cars under our treads. We brought war everywhere we went.

The pace was relentless: a race to an objective, a brief engagement — tanks have a way of ending battles quickly — and then back on the highway. We drove all day and all night, from Basra to Nasiriyah to Diwaniyah, stopping only to refuel. Over the course of a week, I slept 10 hours. No one up the chain of command seemed to care about sleep until one of Charlie Company’s tanks drove off a bridge over the Euphrates in the middle of the night. It settled in the riverbed upside-down, and the four Marines inside died.

We doglegged east to Numaniyah, then continued to push northwest on Highway 6. That’s where my friend Brian McPhillips of Second Tank Battalion was fatally shot in the head, but I wouldn’t hear the news for another two weeks. Information rarely travels laterally in war. I was a few miles from the ambush that killed him when I learned that my platoon would lead the battalion over the Diyala River and into Baghdad.

The bridge was partly destroyed. Two-thirds of the way to the far side, a chunk of the span was gone, leaving pieces of exposed rebar and a clear view to the water below. Combat engineers laid a makeshift bridge over the gap. I asked the engineer lieutenant if it would hold a tank’s weight. “I think so,” he said.

As the platoon commander, I could dictate which tank went over first, but it wasn’t really a choice. It had barely been a week since Charlie Company’s Marines drowned in the Euphrates; I left the hatch of my cupola all the way open, prepared to jump free of the vehicle if it went in the water.

As we came over the crest of the bridge, a man on the far side of the river fired an AK-47 at us. This was inconvenient. I was trying to guide our driver onto the engineer’s bridge while scanning the landscape for other threats; being shot at felt gratuitous.

“Co-ax, fire,” I said.

“On the way,” my gunner replied, and ended someone’s life with the tank’s 7.62-millimeter coaxial machine gun.