Marine Commandant James Amos must make a tough call this year on a program that will define the future Marine Corps: whether to develop and buy the Amphibious Combat Vehicle.

“The Commandant considers a replacement craft for his aging AAV7 Amphibious Tractor to be his number-one priority,” said Gen. Amos’s spokesman, Lt. Col. David Nevers, in an email to me this morning. “He will soon make a decision on the future of the ACV.”

The Marines come ashore from ships and fight their way inland from the beaches. That is what they believe is their military DNA. That’s why ACV is the commandant’s number one priority,

While it may be his top weapons system issue, we aren’t sure how soon “soon” will be. Nevers declined to define it. Given that Gen. Amos didn’t receive the in-depth analysis of aldternatives of ACV options until November, it’s unlikely he can make the much-deferred decision in time to affect the administration’s budget request for fiscal year 2015, theoretically due out next month. (Or he’s made a decision and doesn’t want to telegraph it, which would give time for contractors etc. to influence the decision.)

The Amphibious Combat Vehicle is meant to replace the aging and vulnerable Amphibious Assault Vehicle, aka the LVPT-7, which entered service in 1971, ceased production in the early 1980s, and fought with mixed success in Iraq. The AAV, in turn, is the successor of the famous World War II Amtrac, which revolutionized the military role of the Marine Corps. In layman’s terms, these are swimming tanks that carry Marine Corps foot troops — 24 in the AAV — over water, onto the beach, and deep inland.

The Marines tried to replace the AAV before, with the ambitious Expeditionary Fighting Vehicle. EFV was essentially a water-skiing, transforming tank, able to skim over the water like a speedboat at 30 miles per hour — three times as fast as the AAV — and then reconfigure itself for combat ashore. The idea was a troop transport so fast and long-ranged that Navy ships could launch it from 25 miles offshore, beyond the range of coastal anti-ship cruise missile (ASCM) launchers. But missile ranges got longer, the EFV got more expensive, and the program was cancelled by Defense Secretary Robert Gates in 2011. Ever since, Gen. Amos and the Marine Corps have labored to come up with an alternative.

The Marines face a dilemma with no easy answer. Amos has essentially three options: go high, go low, or go slow.

Go high: a high-speed ACV that meets the ambitious performance goals the Marines hold dear — which critics will immediately declare to be too expensive and doomed to meet the same fate as the cancelled EFV.

Go low: a lower-speed ACV that reduces performance to keep down costs — which critics will immediately argue is too marginal an improvement over the existing AAV to spend money on.

Go slow: a delayed ACV that spends more time in research and development in the hopes of reconciling high speed and low cost — or at least waiting out the current budget crunch.

“Those sound like the generic options to me,” agreed Loren Thompson. “The Marine Corps is not of one mind on ACV, but the path of least resistance at the moment is to keep the effort in R&D.”

But delaying ACV raises near-term dangers, warned Thompson, a well-connected defense consultant, chief operating officer of the Lexington Institute thinktank, and a member of our Board of Contributors. “If the service does that,” he told me, “it will have to rely more on tilt-rotors” — the V-22 Osprey aircraft — “and conventional helicopters to get over the beach.” But aircraft, especially Ospreys, cost a lot themselves, and they can only drop off the riflemen and fly away, not drive them overland under armor.

The problem, said Thompson, is that “amphibious vehicles that lack the agility of a planing design” — the water-skiing approach of the high-speed EFV — “are becoming too vulnerable to perform opposed landings.” Even if anti-ship missiles can’t hit the Navy’s transports before they launch the amtracs, anti-tank missiles can easily hit slow-moving vehicles in the water — and even a non-state “hybrid” force like Lebanon’s Hezbollah militia has effectively employed both types of missiles.

Once ashore, there is the risk of roadside bombs and other improvised explosive devices (IEDs), which took a fearsome toll on AAVs in Afghanistan and Iraq: With 24 riflemen and three crew packed into a relatively lightly armored vehicle, a single well-placed blast could kill or maim dozens of Marines. By contrast, the Army’s M2 Bradley carries only three crew and six or seven infantrymen under much heavier armor. Even the Bradley proved too vulnerable to sophisticated Iranian-built IEDs, however, and the Army wants to replace it with a much heavier Ground Combat Vehicle — which is also likely to be put in R&D limbo.

The danger for both programs is that no amount of R&D can square their respective circles. For the Army GCV, that’s upping protection without excessive weight and cost. For the Marine Corps ACV, that’s increasing speed and, if possible, protection without breaking the bank.

Without some improbable breakthrough in amtrac technology, however, “there’s only two ways to travel: ploughing through the water, as current vehicles do, which limits speed, [or] a giant jet ski,” said Thomas Donnelly, a national security expert at the American Enterprise Institute and a hardcore armored-vehicle advocate. “If you wanna go faster, you have to get up on top of the water,” he told me — and that ain’t cheap.

There are other significant choices the Marines must face that could trade performance for affordability. “There’s the question of what the thing does once it gets ashore,” he said. Does it have a cannon or not? Either answer is legitimate, but there’s no free lunch. And lastly, and also like the Army [programs], how much electricity does the thing have to generate? Is it a charging station for dismounts and their gear?”

A major advantage of the Army’s eight-wheel drive Stryker troop carriers over the older Bradley is that it has almost 50 percent more electrical power, letting it power both more onboard equipment — most importantly IED jammers — and the ever-increasing amount of electronics that modern foot troops carry. But generating big kilowatts requires big engines, as does carrying heavy armor and weapons, and all this costs big bucks.

So Donnelly is deeply pessimistic about the ACV. “Either it’ll turn out to be a replay of the EFV — which I thought was the right vehicle, but isn’t affordable under current budgets — or they’ll drop some of the capabilities to try to make it affordable,” he told me. “Most likely outcome is they’ll opt for a dumbed-down vehicle but still won’t be able to afford it.”

The Marine Corps’ original plan was to power through its procurement of the Joint Light Tactical Vehicle, an uparmored Humvee replacement, as fast as possible to free up funding to buy ACVs in bulk as soon as JLTV was done. That plan seems to have fallen victim to the automatic budget cuts known as sequestration.

“The plan had been to make the Marine buy of JLTV quickly in order to clear the decks for ACV production early in the next decade, but that thinking may have lapsed as budget pressures mounted,” Thompson told me. “Gen. Amos was seriously considering giving up the Joint Light Tactical Vehicle to pursue ACV until budgeteers realized there just wasn’t enough money available.”

Even so, Amos insists the ACV remains the Marine Corps’s top priority for the future force, exceeded only in importance by keeping the current force trained and ready for combat. There had been speculation that Amos was softening his stance and putting the Marine Corps variant of the Joint Strike Fighter, the F-35B, ahead of the Amphibious Combat Vehicle. Not so, said Nevers: ACV is “number one.”

“While the ACV and JSF are at far different stages of programmatic development” — one yet to settle on a basic design, the other already in operational testing — “the capabilities represented by these two platforms are both critical to our sea-based expeditionary mission sets,” Nevers told me.

What Nevers didn’t say is that, as a practical political and budgetary matter, the F-35’s future is secure, supported by Congress, the highest levels of the Defense Department and by three armed services, two of them — the Navy and the Air Force — much larger and more influential than the Marines. A new amtrac is primarily a Marine Corps priority.

Primarily, but not exclusively. While discussion of potential war in the Pacific have focused on a long-range, high-tech exchange of missiles and cyber attacks with China, Marine amphibious forces would be crucial to seize and defend island bases, especially on the flanks of the main “Air-Sea Battle.” And in a lesser conflict, long-range firepower is less important than the capacity to kick the Chinese off a disputed island — and that takes Marines.

If Gen. Amos can convince the Navy, the Air Force, and the Secretary of Defense that a new amtrac is essential to the scenario that worries them most, the ACV program will have a much better chance.

[Corrected 10 January: The original version of the article mistakenly said AAV-7s were deployed to Afghanistan; this is incorrect.]