For almost fifty years, the U.S. Navy has been in command here, using the place as an extensive training area. There are no civilians, but parts of the island are an important wildlife sanctuary. There are lots of rare birds and California sea lions, who don’t seem to care about violent explosions, shells, and naval air landings. Up in the northeast, right on the coast, you find SEALs.

And there we learned the rudiments of fast and accurate combat shooting, the swift changing of magazines, expert marksmanship. We were introduced to the deadly serious business of assaulting an enemy position and taught how to lay down covering fire. Slowly, then faster, first in daylight, then through the night. We were schooled in all the aspects of modern warfare we would one day need in Iraq or Afghanistan — ambushes, structure searches, handling prisoners, planning raids. This is where we got down to all the serious techniques of reconnaissance.

We moved on to really heavy demolition, setting off charges on a grand scale, then hand grenades, then rockets, and generally causing major explosions and practicing until we demonstrated a modicum of expertise.

Our field training tasks were tough, combat mission simulations. We paddled the boats to within a few hundred yards of the shore and dropped anchor. From that holding area, we sent in the scout recon guys, who swam to the beach, checked the place out, and signaled the boats to bring us in. This was strict OTB (over the beach), and we hit the sand running, burrowing into hides just beyond the high-water mark. This is where SEALs are traditionally at their most vulnerable, and the instructors watch like hawks for mistakes, signs that will betray the squad.

We practiced these beach landings all through the nights, fighting our way out of the water with full combat gear and weapons. And at the end of the fourth week we all passed, every one of the twenty trainees who had arrived on the island. We would all graduate from BUD/S.

I asked one of our instructors if this was in any way unusual. His reply was simple. “Marcus,” he said, “when you’re training the best of the best, nothing’s unusual. And all the BUD/S instructors want the very best for you.”

They gave us a couple of weeks’ leave after graduation, and thereafter for me it was high-density education. First jump school at Fort Benning, Georgia, where they turned me into a paratrooper. I spent three weeks jumping out of towers and then out of a C-130, from which we all had to make five jumps.

That aircraft is a hell of a noisy place, and the first jump can be a bit unnerving. But the person in front of me was a girl from West Point, and she dived out of that door like Superwoman. I remember thinking, Christ! If she can do it, I’m definitely gonna do it, and I launched myself into the clear skies above Fort Benning.

Next stop for me was the Eighteenth Delta Force medical program, conducted at Fort Bragg, North Carolina. That’s where they turned me into a battlefield doctor. I suppose it was more like a paramedic, but the learning curve was huge: medicine, in-jections, IV training, chest tubes, combat trauma, wounds, burns, stitches, morphine. It covered just about everything a wounded warrior might need under battle conditions. On the first day I had to memorize 315 examples of medical terminology. And they never took their foot off the high-discipline accelerator. Here I was, working all day and half the night, and there was still an instructor telling me to get wet and sandy during training runs.

I went straight from North Carolina to SEAL qualification training, three more months of hard labor in Coronado, diving, parachute jumping, shooting, explosives, detonation, a long, intensive recap of everything I had learned. Right after that, I was sent to join the SDV school (submarines) at Panama City, Florida. I was there on 9/11, and little did I realize the massive impact those terrible events in New York City would have on my own life.

I remember the pure indignation we all felt. Someone had just attacked the United States of America, the beloved country we were sworn to defend. We watched the television with mounting fury, the fury of young, inexperienced, but supremely fit and highly trained combat troops who could not wait to get at the enemy. We wished we could get at Osama bin Laden’s al Qaeda mob in Iraq, Iran, Afghanistan, or wherever the hell these lunatics lived. But be careful what you wish for. You might get it.

A lot of guys passed SEAL qualification training and received their Tridents on Wednesday afternoon, November 7, 2001. They pinned it right on in a short ceremony out there on the grinder. You could see it meant all the world to the graduates. There were in fact only around thirty left from the original 180 who had signed up on that long-ago first day of Indoc. For myself, because of various educational commitments, I had to wait until January 31, 2002, for my Trident.

But the training never stopped. Right after I formally joined what our commanders call the brotherhood, I went to communication school to study and learn satellite comms, high-frequency radio links, antenna wavelength probability, in-depth computers, global positioning systems, and the rest.

Then I went to Sniper School back at Camp Pendleton, where, unsurprisingly, they made sure you could shoot straight before you did anything else. This entailed two very tough exams involving the M4 rifle; the SR-25 semiautomatic sniper rifle, accurate to nine hundred yards; and the heavy, powerful 300 Win Mag bolt-action .308-caliber rifle. You needed to be expert with all of them if you were planning to be a Navy SEAL sniper.

Then the real test started, the ultimate examination of a man’s ability to move stealthily, unseen and undetected, across rough, enemy-held ground where the slightest mistake might mean instant death or, worse, letting your team down.

Our instructor was a veteran of the first wave of U.S. troops who had gone in after Osama. He was Brendan Webb, a terrific man. Stalking was his game, and his standards were so high they would have made an Apache scout gasp. Working right alongside him was Eric Davis, another brilliant SEAL sniper, who was completely ruthless in his examination of our abilities to stay concealed.

The final “battleground” was a vast area out near the border of Pendleton. There was not much vegetation, mostly low, flat bushes, but the rough rocks-boulders-and-shale terrain was full of undulations, valleys, and gullies. Trees, the sniper’s nearest and dearest friends, were damn sparse, obviously by design. Before they let us loose in this barren, dusty no-man’s-land, they subjected us to long lectures stressing the importance of paying attention to every detail.

They retaught us the noble art of camouflage, the brown and green creams, the way to arrange branches in your hat, the dangers of a gust of wind, which might ruffle your branches alone if they weren’t set tight, betraying your position. We practiced all the hours God made, and then they sent us out onto the range.

It’s a vast sweep of ground, and the instructors survey it from a high platform. Our stalk began a thousand yards from that platform, upon which the gimlet-eyed Webb and Davis stood, scanning the acres like a pair of revolving radars.

The idea was to get within two hundred yards of them and then fire through the crosshairs at the target. We had practiced doing this alone and with a partner, and boy, does this ever teach you patience. It can take hours just to move a few yards, but if the instructors catch you as they sweep the area with high-powered binoculars, you fail the course.

For the final test I was working with a partner, and this meant we both had to stay well concealed. In the end, he finds the range and calls the shot, and I adhere to his command. At this stage the instructors have installed walkers all over the place, and they’re communicating by radios with the platform. If the walker gets within two steps of you, you’ve failed.