NATIONAL HARBOR: Cheap grey-market missiles and commercially available radar kits are forcing the Marines to reinvent amphibious warfare for the 21st century. The new Corps concept, Expeditionary Force 21, predicts long-range threats will force the fleet to stay at least 65 nautical miles offshore, a dozen times the distance that existing Marine amphibious vehicles are designed to swim. The ramifications are just beginning to ripple across tactics and technology, starting with a radical overhaul of the Marines’ Amphibious Combat Vehicle program and a new emphasis on high-speed landing craft.

As longer-ranged precision weapons proliferate to potential adversaries around the world, “we’ve moved from the high-water mark to 12 nautical miles [offshore] and now we’re at about 65 nautical miles,” said Assistant Commandant of the Marine Corps, Lt. Gen. John Paxton, when I asked him about EF-21 after his public remarks at last week’s Sea-Air-Space conference.

“That’s the anticipated threat ring in the near- to mid-term,” Paxton told me. (Emphasis mine). “The range is going to continue to go further out.”

In fact, anti-ship cruise missiles can hit ships 165 nautical miles from shore, according to a Marine Corps briefing slide, while China’s still-in-development anti-ship ballistic missile can go at least 300 miles. (No figures yet on China’s prototype hypersonic missile). According to the slide, the 65-mile figure simply reflects the maximum range of the Chinese-made C-802 Silkworm.

The C-802 is widely available — the Lebanese militia Hezbollah used one to cripple an Israeli corvette in 2006 — but it is hardly cutting-edge. Conversely, at closer ranges, even a low-end adversary without anti-ship missiles can get cheap sensors to find the fleet and direct, for example, swarm attacks by high-speed boats or even use artillery, precision-guided mortars, and anti-tank missiles.

Those sensors matter as much as the weapons, and they’re even more widely available, said retired Marine Col. Doug King, director of the Commandant’s “Ellis Group” of hand-picked thinkers: “You can buy a 25-nautical-mile radar; they’re everywhere.”

I did a Google search and a commercial brochure on just such a radar was my first result: “Now you can see crystal clear targets up to 32 nautical miles away.” The radar weighs less than 17 pounds and I could have it shipped to me tomorrow for $1,999.

So a 65-mile standoff from shore is a general guideline, not a magic number, King said. It’s a safe rule of thumb to follow when “we’ve arrived on scene, we’re not certain of the A2/AD [anti-access/area denial] threat, and we need to put Marines ashore,” he said.

As threats proliferate, King told me, the Marine Corps face a choice:

“If the United States is in trouble or there’s a citizen that is in trouble… are we going to ‘shape’ that environment, [i.e.] basically defeat all the [threat] systems so we can come in close to the two miles or the 12 miles [from shore] that people thought of before, or are we going to respond to the crisis?” King said. Under Expeditionary Force 21, he said, “our goal — and we can do this today — is we can come from a greater distance.”

Expeditionary Force 21 will be updated every year. So how the Marines will keep changing the concept. But listening to Marine leaders like King makes two things clear:

they don’t have any illusions that some magical safe zone lies beyond the 65-mile mark, and

they won’t wait for US air, sea, and cyber power to blast a safe path to the shore.

Indeed, in at least some cases, they see small landing teams — much like the Marine Raiders of World War II — as the cutting edge of the joint force, slipping in by boat or V-22 aircraft to destroy key enemy sensors or anti-ship weapons so the larger force can approach.

“If the Navy’s unable to get closer in because of the threat, we just can’t sit there and wait,” Brig. Gen. William Mullen said at Sea-Air-Space. The challenge is getting Marines through the threat zone alive.