The Marine Corps’s current Amphibious Assault Vehicle, the 1970s-vintage AAV-7.

Imagine you’re a military supply officer, weary but proud as you watch the train you’ve laboriously loaded with gear roll out of the depot towards the front. And then you realize: You packed the wrong tank. Now you need to get that vehicle off and the right vehicle on — while the train’s already leaving the station.

Stressed yet?

That’s how the Marines must feel right now as they scramble to shift funding in a fiscal 2015 budget request that’s due out March 4. Specifically, they need to reallocate, repurpose, or at least rename funds currently budgeted for their ambitious Amphibious Combat Vehicle, which they’ve had to postpone, and transfer them to a more modest Marine Personnel Carrier designed to meet the service’s immediate needs.

The agony is that the Amphibious Combat Vehicle was, until last month, the Marine Corps’ top-priority program, the holy grail of a 25-year quest to replace slow and vulnerable 1970s-vintage AAV-7 amphibious transports. The irony is that just last year, the Marines effectively killed the Marine Personnel Carrier, explicitly to free up funding for the ACV. Just weeks ago, however, Marine Corps Commandant James Amos decided he had to reverse course, postponing ACV indefinitely while reviving MPC as a quick and partial fix– after the Marines had already submitted their 2015 budget.

Whatever’s in the name, the new vehicle will not be the rose the Marines had long hoped would bloom. It will be something much more limited — but one that is much more achievable. And, as the commandant himself has said, the military is now in an era of “good enough.”

Gen. Amos has spent his entire term making painful choices about the Marine Corps’ top priority: a new, faster, and better-armored amphibious troop transport to carry riflemen from ship to shore and then drive on inland. That’s a mission currently performed by vulnerable 1970s-vintage Amphibious Assault Vehicles called AAV-7s (formerly LVTP-7s), which direly need replacing. But with that replacement, the ACV, on hold, the Marines need to fund something else fast: The Marine Personnel Carrier.

“This decision was made almost the day after the Marines submitted their budget,” one knowledgeable defense official told me, on condition of anonymity because of the “angst inside the Marine Corps” over the matter. “There’s all kinds of things you have to do to restart, [but] the money problem is our biggest issue right now.”

“Both the bureaucracy in the Department and on the Hill appear to be supportive,” he went on. At this late stage, it’s far from easy to “go through all the bureaucracy, the political wickets, to see if we’re capable of shifting this money from ACV into what was MPC.”

That said, “I think you won’t ever see it called MPC,” the source went on. “Instead, “you may hear it called ‘ACV phase 1′” — even though it won’t actually be a fully amphibious Amphibious Combat Vehicle capable of transporting Marines from ship to shore.

The Marines’ first attempt at the replacement was a kind of water-skiing tank called the Expeditionary Fighting Vehicle. But EFV grew so complex and costly that then-Secretary of Defense Robert Gates cancelled it in 2011, just months after Amos became commandant. Back then, Amos vowed he’d be driving a new, more modest, and more affordable Amphibious Combat Vehicle (if only in prototype) by the time he finished his term as top Marine — which is later this year.

But Gen. Amos recently reviewed the service’s in-depth studies one last time and decided that budgets were too tight and the technology too immature to develop an ACV that had the desired performance both on water and on land. Instead, he announced a two-phase approach: The service will buy some kind of interim vehicle to supplement the aging AAVs in the near term while it continues development on a future vehicle that combines high water speed with onshore fighting power at an affordable cost.

Now it turns out that phase one will be a revived — and probably renamed — Marine Personnel Carrier.

The Marine Personnel Carrier is not and was never meant to be a substitute for the Amphibious Combat Vehicle. The Marines had in fact originally planned to buy both as complementary vehicles for different missions. In the first wave, a limited number of expensive, fully amphibious vehicles — originally EFVs, then ACVs — would carry a spearhead force from ship to shore and on inland. Later, as reinforcements, a larger number of cheaper Marine Personnel Carriers would arrive to transport troops who didn’t have ACVs. The MPC would need some amphibious capability, but only enough to cross rivers and other water obstacles common in coastal zones, not enough to cross the miles of sea from an amphibious ship to the beach. Instead, the Marine Personnel Carrier would have to be itself carried ashore on some kind of landing craft.

As budgets tightened, however, the Marines decided the fully amphibious ACV had to be their urgent priority and zeroed out the funding for the more limited MPC. After all, they reasoned, there are plenty of armored, wheeled troop transports on sale for reasonable prices from companies around the world: When we finally can spare the cash, we can easily buy one off the shelf.

But the budget kept getting smaller at the same time as the ACV’s technical challenges kept looking bigger. Meanwhile, MPC trials held last summer showed that wheeled armored personnel carriers “have come a long way” in the amount of protection and mobility they can provide, the defense official said. So while the MPC vehicles can’t solve the whole problem, they can solve a big piece of it — and you can buy them now.

In the long term, of course, the Marines still want a way to move swiftly over long distances from ship to shore. That’s what Amos’s Phase 2 is all about. But when and what will it be?

That’s wide open, the defense official told me. In fact, he said, “I’m not so sure that second phase means a high-speed amphibious vehicle” — the goal the Marines have been pursuing since at least 1988.

“A lot of Marine senior leaders [think] we may just have been chasing the wrong vehicle,” he said. That faction is “by no means the majority,” he made clear. But the cancellation of the EFV in 2011 and the indefinite delay of the ACV this year have convinced at least part of the Marine Corps that they’ve gone down a technological and budgetary dead end.

Instead of a single vehicle that both moves at high speed across the surface of the water and fights on land, there’s a new openness to a two-piece solution: say, for example, a troop-carrier optimized to operate on the land, with limited amphibious capability to wade through rivers and surf, and a high-speed watercraft to bring that land vehicle to the beach, or at least close enough to dog-paddle there.

In the meantime, the Marines really want to get going on the new Marine Personnel Carrier. “The ’15 budget is critical to allow [it] to get started,” the official told me. “Even if we started in ’15,” he added, the time to hold a proper competition and evaluation and then start buying in quantity would mean “the Marine Corps doesn’t see operational quantities of those vehicles until fiscal year ’20 or ’21.”

What about the future high-water-speed solution, Amos’s Phase 2? “Honestly, I couldn’t even begin to figure out the timeline for that,” the official said.