Every Memorial Day, a massive invasion force descends on the island of Manhattan.

I'm a war correspondent for Popular Mechanics, on board the amphibious assault ship USS Bataan. We're on a high-profile military mission and I'm in the ready room, waiting for our pre-mission brief, as dozens of chiseled Marines wearing desert tan battle-dress uniforms file in. These guys could just as easily be walking into class on some college campus, but instead have chosen to major in defending the Constitution against all enemies foreign and domestic. I'm embedded with infantry Marines from Camp Lejeune's 24th Marine Expeditionary Unit, a badass unit designed to be a "forward-deployed, rapid-response force capable of conducting conventional amphibious and selected maritime special operations at night or under adverse weather conditions from the sea, by surface, and/or by air." They are trained to "locate, close with, and destroy the enemy by fire and maneuver."

The last time I took notes at a pre-deployment brief was back when I was around their age, in 2003, just before heading into Iraq with my own infantry unit. A slight case of déjà vu sets in as a well-built public-affairs officer, a captain, walks to the front of the room and sets up his PowerPoint presentation.

"Gentlemen," he asks, "what do you think your mission is going to be?"

One Marine answers, "Hearts and minds, sir?"

I've been on one of these before, I think to myself. Our mission to Iraq was to win hearts and minds, too. And to "locate, capture, and kill all non-compliant forces."

The captain discusses rules of engagement and provides the young combatants phrases to avoid when communicating with the natives. Examples given: "I'm looking forward to getting free beer," and "I honestly don't know what we're doing here."Talk of politics is also a no-no.

These Marines are going to be facing off against a group of people renowned for hostility and abrasiveness, in a place famous for its harshness and lack of humanity, a place governed by chaos and overrun with vermin, where the people drive their vehicles like weapons: New York City. Their mission: Fleet Week.

Marines winning hearts and minds at the Raccoon Lodge. Mark Peterson

During this brief the men are shown combat footage from previous Fleet Weeks—first an unbelievable clip of some Marines getting their butts kicked in a tug-of-war with the Navy on Fox & Friends, another of two inebriated Marines trying to talk to TMZ about what it was like to crash a David Letterman party. Oh, the humanity.

The public-affairs officer explains that he's sharing this footage because, although sailors and Marines on leave have been known to do much good during Fleet Week—work with Habitat for Humanity, the Girl Scouts, and Meals on Wheels is the stuff he'd prefer to publicize—all it takes is one moment of poor judgment to muck it all up. I always thought Fleet Week was just an excuse for sailors and Marines to get "stewed, screwed, and tattooed," as they say, but the way the captain is going on with a straight face about the goals of this mission—"to kill negative perceptions," etc.—it seems I may have been misinformed.

Has it always been this way during Fleet Week? The first celebratory deployment to New York was more than a century ago, and now half a dozen other cities—San Diego and San Francisco among them—also host parts of the Naval fleet.

Come on New York, show these boys a good time. By the time you read this, they will be deployed.

I put down my pen, take off my Wayfarers and rub my eyes, mumble a humble prayer to the gods of pleasure: Come on New York, show these boys a good time. By the time you read this, they will be deployed, back to kill or be killed in the same war I fought so long ago.

On the ship, en route from Norfolk, Virginia, I thanked God I was never a Marine. I couldn't have handled it. The constant rocking back and forth feels as if you're always walking home after happy hour, and with all the steel and sharp angles and cramped quarters, it feels as if you're stuck in a floating prison. There's no escape, no Wi-Fi, no cellphone reception. Everyone's cut off completely from the world, their families, and their social media accounts. Belowdecks, where the grunts live: Marines in boxers, decorated with various USMC tattoos, walk around spitting tobacco juice into water bottles. Guys playing spades, ironing boards with uniforms stretched tight, a half-dozen Marines crowded around a laptop watching a bad action movie. The bunks are stacked, there's hardly any room and no privacy whatsoever.

Back up on deck, the sun's out and it's a beautiful day on the Hudson, so perfect that it's as if the war's been won and we're all headed to the victory parade in the urban canyons. It's a celebratory atmosphere as we approach H-Hour on D-Day. We disembark and establish a beachhead at Pier 88 on the western front of Manhattan, one mile from Times Square. While waiting for the ramp to drop so that the amphibious assault can begin, we're informed that the first wave of sailors and Marines to be dismounted will appear on the Jimmy Fallon show, the second wave to a barbecue out in Brooklyn.

The invasion is underway.

In the ready room of the USS Bataan, preparing to deploy. Mark Peterson

Before long, night has fallen and it's chaos. Bodies everywhere. Mark the photographer and I are in some kind of alley near Wall Street, packed with bars with young, well-dressed, urban professionals mingling. Friend or foe, hard to tell. The bars overflow, filling the outdoor seating, too. Everyone is having a great time. We can't find our Marines. Just then, I come across Corporal Joe Kruggel, one of our guys, standing outside a bar. He has a kind smile, pressed uniform, and athletic features. A movie star version of a Marine. As he guides me into the bar to meet his squad, I see a daisy chain of about six Car Bombs come into view, lined up on the bar waiting for the Marines. Corporal Jose Rivera from South Gate, California, intercepts the Car Bombs and hands them out to the men. He's a quiet guy, and passes me one of the drinks just as Corporal Andrew Lawrence from South Carolina swoops in for one. Lawrence gets doubletakes from some girls seated nearby, as does Corporal Marion Woodward from South Carolina, a tall fellow who wears his blond hair slicked back. They're all in uniform and looking fine, a regular Marine Corps Rat Pack, and this is one of those crystalline moments of peak life you might see faded in a frame in a den somewhere when these guys are old.

They're all around the same age—early twenties—all enlisted, having graduated infantry school at the same time. Not only that, they've all extended their enlistment so that they can be deployed together. None has ever been to New York before. There are several other Marines at the bar, all machine gunners in a weapons platoon. The exact same job I had in the Army. It's been years, but from memory I start reciting the characteristics of the M240 Bravo machine gun to Kruggel: The M240B is a belt-fed, air-cooled, gas-operated, fully automatic machine gun that fires from the open bolt position . . . It weighs approximately 27.6 pounds . . . Can be fired on a tripod, bipod, or vehicular mount . . .

As we're talking about maximum effective ranges, cleaning procedures, rates of fire, and how absolutely awesome the M240 Bravo is, Kruggel's phone rings. He quickly picks up and says, "I'm at a bar. Love you." Click. He shoves his phone back in his pocket and we go back to gun talk for quite some time. A little later, I ask who that was earlier on the phone.

"Oh," he says. "My wife."

The Marines and I go through many more Car Bombs and many more topics like women in the infantry, what they think of liberals, talking mad smack about non-infantry soldiers, or POGs (Persons Other than Grunts). None of them are censoring themselves and I start to see flashes of my old platoon mates in each and every one of them. Woodward pulls me aside to tell me how a public-affairs officer had been extremely nervous about me following them around during Fleet Week, so much that they had threatened to assign an officer to make sure they were on their best behavior. Must have been the hearts-and-minds captain. But as I go back to assessing the battlefield, what I see before me would make the greatest recruiting poster ever conceived: four young, beautiful women seated at the bar to my right, each deep in conversation with at least one Marine. To my left are a couple cocky-looking investment banker types, bro-ing out with their Marine counterparts. In theater, these Marines are without fear.

Before long, every single one is making out with the lady he was flirting with earlier. One girl is wearing a Marines' white dress hat. During a break in the action, I approach one of the women just as her friend with the hat takes a selfie to post on Facebook. I ask, "So Marines, huh?"

"Omigod! Marines are hot!" Mark Peterson

"Omigod!" she says. "Marines are hot."

I realized I had made a grave mistake. I should have been a Marine.

Day plus one, everybody is still alive and nobody spent the night in jail. It's around 13:00 and I'm at the bar at the Hard Rock Cafe in Times Square. Seated next to me is Corporal Kruggel, who has his cellphone out. When he shuts it off, I see his screensaver is of a woman. His wife, Sam.

Kruggel grew up on a dairy farm in Ohio, a skater kid with long hair who milked cows. He's known his wife since sixth grade but never said a word to her until the end of senior year—she was a bit preppy and hung out with the cool kids. One day, he asked for her number so that he could take her to Chipotle, and she agreed. They hit it off. They're both twenty-one now, having married at eighteen. She was cool about him joining the Marines.

"I'm a weirdo in the civilian world," he says. "But I'm less weird in the Marine Corps, if that makes any sense?"

The only hard part is being away from Sam so much. Kruggel stares longingly at her picture. She's never been to New York City before and she and a friend are driving all the way from Ohio to be here. She's a bartender at an ale house and he tells me he's extremely proud of her. "She's been working double shifts and saving up money so that she can afford the trip and stay at a hotel since it's so expensive here."

Back in action, the Marines can't go five yards without being approached by someone wanting to have a photo taken with them, without someone wanting to shake their hands, without "Thank you for your service." That last one became hard to hear back when I was returning from war. And may be even harder to explain. But it had something to do with the serviceman-as-prop, a quick "thank you" absolving all sorts of sins. Which makes one ask: Are we a country at war? None of these Marines has experienced war yet. New York doesn't seem like a city in a country at war, either. But, that's always been the case. At least since my return from Iraq in 2004.

It's Memorial Day and all weekend long, each time I check Facebook, there's an old platoon mate posting photos of guys we knew who never made it back. I'm not sure if I should press "like" on them, but they're there and I can't help but think how that could have been me.

In Times Square, we pass an Armed Forces recruiting station, which is completely empty, and enter McDonald's, which has a huge line. Once seated with our meals, the topic of discussion is where to drink next. Anywhere with free drinks seems to be the preferred option, so everyone pulls out their cellphone to pinpoint locations where those in uniform during Fleet Week drink on the house, as if they're trying to pinpoint mortar rounds on an ISIS position. Within seconds they find several nearby—the problem being they don't start serving for a couple hours.

The CO expresses his appreciation for an old Marine at a Memorial Day Parade. Mark Peterson

With time to kill, we leave in search of any old bar, but the Marines discover the Times Square Hershey's store and want to check it out. They take selfies with the oversize jars of chocolate syrup and stare in awe at giant chocolate bars. Then they spot the two-story M&M's store across the street and are equally excited about that. It is in the candy stores that it hits me: They might be Marines, but they are still just boys—sweet, enthusiastic kids. Out the door and they're swept up in the swirling choreography of Times Square. Wow, Bubba Gump's! Wow! The national debt clock! Whoa! The Naked Cowboy! Whoa! The Naked Cowgirl!

As we're walking, a man runs up to a Marine and hands him a twenty-dollar bill, which the Marine refuses to no avail. As we pass the exact location of that iconic World War II victory kiss, a young woman runs up behind Kruggel to give him a kiss on the cheek before he can stop her, then quickly disappears into the crowd. He is red with embarrassment and immediately Kruggel worries out loud about what his wife would think.

Next to Morgan Stanley's midtown office tower is an Irish bar, Hurley's Saloon. A sign outside advertises three-dollar drinks for service members. I worry the guys are walking into an ambush when inside I see all the men in striped, white-collared button-downs, a rainbow of Brioni ties—one guy's wearing a bow tie, for chrissakes. All the ties turn to stare. The first shot is fired when Jimmy, who talks like a guy in a Scorsese movie, sizes up the Marines and tells the bartender, in his thick Brooklyn accent, to get them all a drink, and "Put 'em on my tab."

Once all the Marines have a beer in hand, Jimmy asks them where they're all from. It gets super tense for a second when one of them nervously responds, "Boston, sir."

"Put 'em on my tab," Jimmy says. He was in the WTC on 9/11. On the day the towers fell, these Marines were all little kids.

"Boston?!" Jimmy yells, laughing. "Well, I don't know about you. Gimme that beer back!" Thanks to Jimmy, it's now a party. He's massive not only in size but personality. He's constantly checking in on every Marine to make sure each is set with a full beer in hand.

When I ask him why he's so welcoming to the Marines, he tells me, "I was there—I was in the Towers when they got hit, up on the sixtieth floor." As he's starting his story, I grab Kruggel: "Listen to this."

Jimmy was in the World Trade Center for the '93 bombing, too, and on 9/11. "I knew this was it," he says. Before leaving the building, "I had to go back and grab my trading notebook," which his friends will give him eternal grief for, but he says he couldn't leave without it.

Pensive Marines at Ground Zero Mark Peterson

Another Brioni with a New York name—Vinnie—and an accent to match comes over. He works with Jimmy and was there on 9/11, too. He remembers going down the stairs with a buddy and saying to him, "It was nice knowing you." The bar gets quiet as the Marines all gather around, listening intently. The day the Twin Towers fell, they were all little kids.

At Ground Zero the next day, the Marines are once again celebrities, with families asking for photos, asking if their kids can try on their hats.

As we look down into the memorial—two large, square holes in the ground where the towers once stood, water running down into them—Kruggel asks if it's true, that the dust and ash rose from Ground Zero for months. I tell him yes, that it would just drift across the city. He looks up at the sky, as if he might still see the floating particles.

On the last night, Kruggel and his buddies are a couple blocks from the USS Bataan, and their 23:00 curfew looms, less than an hour away. Tomorrow, the Marines head back to Norfolk, and from then on everything they do will be in preparation for their coming deployment. They've attacked this mission with everything they had and then some. One man has a bandage on his head from some hand-to-hand combat the night before. The mood is reflective. Party's over.

As the clock ticks down to the sober straight-and-narrow, the Marines drink a quiet beer and talk about Iwo Jima. Today they got to meet a veteran of that battle, and they're all in agreement that it was the best few hours of the past week. Kruggel, awestruck, has a picture on his phone of the whole group gathered around the old man, who was their age when his country called him to the Pacific. His best friend had been blown to bits within five minutes of landing on the beach, and he told them how the flag had been raised on top of a mountain, followed by some weary clapping, and then they went right back to fighting. Rivera tells me that after the old soldier shared his story, the man's daughter said that it was the first time she'd ever heard him talk about that day. My father was the same way. Army infantry, during the Tet Offensive. Silver, Bronze stars and a couple Purple Hearts—all of which I learned about while rummaging through storage boxes in the garage.

"What would you guys be doing if you hadn't joined the Corps?" I ask.

Lawrence and Woodward both say college. Lawrence says that one day, if he gets out, he'll use his GI Bill to become an architect. Rivera says he'd probably be a cop, since he likes helping people. When I get to Kruggel, Sam, who showed up a couple days ago—the woman actually drove the sixteen hours from Litchfield, Ohio, to spend these last few hours with her man before he sails back to Norfolk—and is just radiant, jumps in. "Oh my God!" she says. "Milking cows!"

Saying goodbye on the last night as curfew approaches Mark Peterson

Cradling his beer, Kruggel smiles and thinks it over. He nods, takes a sip, and says, "Yeah, that's why I love the Marines."

Outside the bar we shake hands and I wish them luck, and they disappear down the dim street. It's almost midnight, and I walk the quiet streets to the Blue Ruin, a hard-knuckles dive in Hell's Kitchen. I'm the only one there and I can't help but notice the small American flag that someone left on the bar, and the large Marine Corps flag hanging on the wall. I order a shot and a drink for me and for the bartender, too, and we drink to the safety of the 24th Marine Expeditionary Unit, a badass unit designed to be a forward-deployed, rapid-response force. Salud, boys.