I soon made friends with the local skaters: Big Clive, a Jamaican kid from Brixton; Toby Shuall, the son of an Israeli jeweler; James (Paddy) Neasdon, whose uncle may have been in the I.R.A. I became Fat Yank. (The lean Marine with the buzz cut came later.) We spoke our own language. Skateboarding tricks — backside tailslide, varial kickflip. Girls — She’s a total Bettie. Put-downs — Don’t be a T-Dog, Land the trick. And my favorite: Safe.

Image Elliot Ackerman Credit... Peter van Agtmael

Safe meant cool. It meant hello. It meant don’t worry about it. Once, when trying a certain trick on the beam, as we called it, a long wooden ledge topped with shards of granite, I toppled onto the stones, damaging a nerve in my hand — my right index finger still tingles when it’s cold out — and Toby came over, helping me up: Safe, man. Safe. A few minutes later, when I landed the trick, my friends banged their boards on the ground, shouting: “Safe! Safe! Safe!” And that’s what mattered — landing tricks, being a good skater.

When I was 15, my family moved to Washington. I tried skateboarding there, but the locals were far less welcoming. Within a couple of years, I’d given it up. It was time to start thinking about college, the future. My parents were surprised when I decided to join the Marines, but the draw soon became obvious. The insider’s language — The enemy’s T.T.P. is to use frags. The varied backgrounds — my first platoon had kids from Texas, the Dominican Republic and Canada within its ranks. And the importance of being good, or as Marines say, tactically competent.

Over eight years, I deployed five times to Iraq and Afghanistan, serving in the infantry and a couple of special-operations units, some of them sustaining more than 50 percent casualties. Friends like Staff Sgt. Sean (the Skwerl) Brownlee and Gunnery Sgt. Willy (Bare Knuckles) Parent became dear comrades, replacements for my skater friends. I lost other friends then, a roll call of ghosts who follow me still.

When I returned to London in 2004, I found myself wandering down to Southbank, spending hours there. I’ve traveled back several times since, most recently this past spring. The day was cold but clear; tourists and Londoners stopped to watch the skaters. I mixed among the onlookers but wanted to get closer. Weaving among the kids who rushed by on their boards, I found my way to the beam, sitting on the petrified piece of wood, running my hands along the shards of granite cemented to its top. I could still feel the tingling in my finger, the one I’d stopped thinking of as my right index finger and had, some years before, started thinking of as my trigger finger.