Here's what happens when US Marines storm an enemy beach

On Thursday, US Marines with the 15th Marine Expeditionary Unit embarked on a training exercise, carrying out an amphibious assault on a town littered with notional insurgents — and as should be expected from a premier maritime force, the grunts dominated.

Business Insider was on hand to watch as Marines left their ship, the USS San Diego, and headed onshore, practicing landing on and taking over an enemy beach.

Here's what we saw.

It's a beautiful day to check out the beach in Southern California.

But today there's something lurking off the coast: The USS San Diego, an amphibious transport packed with thousands of Marines and sailors from the 15th Marine Expeditionary Unit.

The infantry grunts in the main assault are with Battalion Landing Team 1/5. Cpl. Rafael Anderson, a squad leader, has the battalion motto of "Make Peace or Die," on his flak jacket.

As part of a predeployment training scenario, the Marines' mission is to assault the beach, capture a high-value target, and pacify this town, which has been taken over by what officials say is a "generic" opposition force.

Though most of the opposing force is dressed like insurgents from the Middle East, Marine Pfc. Raul Galvin brought his Air Jordans to the fight.

Many others dressed as insurgents have taken up positions in the town, awaiting the Marine assault.

Having an enemy to oppose the Marines is important, as Capt. Andrew Paul explained, since officials want this training exercise to be as close to the real thing as possible. "The more realistic we can make it, the better," he said.

Once Marines get the command to launch, they drive their amphibious assault vehicles off the back of the ship into the water. These 29-ton monsters are able to float.

The Marines also launch aviation assets over the town, to include Hueys and Cobra gunships from VMM-161.

With about 20 Marines in the back and a crew of three, the AAVs lumber through the water at about 8 mph.

Then they finally hit the beach.

The AAV drivers push up the beach to the drop-off point, where Marines will eventually get out and move on foot.

Others follow suit.

And obstacles are not a problem.

A couple of minutes after the AAVs land, the drivers stop and the ramp is dropped. Marine infantrymen start piling out.

One of the sergeants looks on to ensure his Marines are getting out of the AAV and moving into the assault.

In the real world, the grunts would most likely be supported by the AAV's weaponry, which is typically a .50-caliber machine gun or an Mk-19 automatic grenade launcher.

And the grunts run like crazy up the hill and into the town.

"They did really well across the beach," Lt. Col. Richard Alvarez, the MEU's executive officer, said.

But landing on the beach is only step one. Now the Marines have to methodically move from house to house in the town and clear it of insurgents.

And they are pretty much everywhere.

Besides small arms, the insurgents have mortars ...

... and rocket-propelled grenade launchers. The weapons aren't real, so observers on the training will pick some Marines in the assault to become notional "casualties" whom their buddies will have to care for.

The Marines call this "military operations on urban terrain" — MOUT — and it's typically the toughest kind of fight. Urban fighting like this often makes casualties inevitable.

So it's vital that Marines move fast and look in all directions for potential threats.

In buildings like this, two or three Marines can quickly flood into a room ...

... and take on any insurgents lying in wait.

The bad guys had plenty of their own notional casualties.

Typically, the grunts would clear a building and then face out the windows to take out any threats.

With their fellow Marines covering their back, fire teams move on to other buildings.

While grunts are busy doing their house-to-house fight, the Navy hits the beach with an even bigger craft: a Landing Craft Air Cushion, a huge hovercraft that can transport in larger vehicles and more Marines.

After a good parking spot is found, sailors hop out to help offload more AAVs ...

And Humvees armed with .50-caliber machine guns.

The grunts, meanwhile, need to keep moving.

But the bad guys don't make it easy. Some use civilian women as human shields to move throughout the town.

Others pretend to be dead, waiting for an unlucky grunt to turn the corner.

And at one point, the grunts encounter an armed insurgent hiding behind a woman. "S---, he's got a hostage," one Marine yells. "Put the f---ing weapon down."

The Marines hold their fire, and the insurgent makes it into the compound. But they burst in and clear it soon afterward.

Some insurgents are captured ...

... but many more are killed.

After about 90 minutes of fighting, the Marines secure the town.

And the AAVs park in a 360, so they can watch for any more threats from outside the perimeter.

Camp Pendleton's Red Beach is secure for today ...

But perhaps, as the 15th MEU deploys later this year to the western Pacific — potentially in support of Central Command in Iraq and Syria — the Marines training on this beach may be tasked with pulling off a scenario like this for real.

Here's what happens when US Marines storm an enemy beach

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