The four-day fire aboard the USS Bonhomme Richard has put the ship out of action for months, perhaps permanently. It came at a time when Secretary of Defense Mark Esper and Congress are wrestling over the Navy’s 30-year shipbuilding plan.

An amphibious assault ship, the Bonhomme Richard was nearing the end of a two-year upgrade to integrate the Marines’ F-35B into its capability. The (at least temporary) loss of the ship reduces the Navy’s ability to deploy the F-35 in the Pacific. It makes more fragile the Navy’s short and long-term plans to deter China in the Pacific.

The ship is important, but not essential, to performing the Navy’s part of our national security strategy. What becomes of it is less important than where our military leaders are aiming to go in the next decade and further into the future.

In February, reportedly troubled by the Navy’s stodginess in planning, Esper withheld from Congress the Navy’s 30-year shipbuilding plan. That plan, mandated by Congress, requires the Navy to project its shipbuilding plans and needs for three decades.

It’s nothing short of delusional to pretend you know what ships the Navy will need in 30 years to deter or defeat the threats that will exist then. But because ships, aircraft, and many other weapon systems take so long to design and build, the Pentagon at least has to try to guess what will be needed.

It takes too many years to design and build ships such as the Bonhomme Richard, longer for ships such as our largest aircraft carriers and submarines. Combat aircraft evolve just as slowly. The length of time it takes to design, build, and deploy weapon systems conflicts directly with the rapidity with which threats evolve.

The military services and the procurement laws Congress has imposed on them, coupled with the complexity of needed weapon systems, is often mixed into a toxic brew of bad decisions about what is needed.

It’s not only the Navy: the Army and Air Force have similar problems and have made the same kind of enormous mistakes. Two cases in point are the Army’s “Stryker” wheeled combat vehicle and the Air Force’s top-line fighter, the F-22.

About 15 years ago, I interviewed an Army lieutenant general who was in charge of its program to design and build the “Stryker” combat vehicle. The Stryker is supposed to be deployed quickly to places such as the Middle East. I asked the general why it wasn’t designed to be adequately protected against rocket-propelled grenades, which are so common in the Middle East that they are a basic fashion accessory. He told me — in all seriousness — that the Army wouldn’t deploy the Stryker in such places.

And that’s not all. To deploy Army forces quickly is the Air Force’s task. But the Stryker is too heavy and too large to fit into our primary tactical airlifter, the C-130. What on Earth was the Army thinking when it designed the Stryker that way?

Our stealthiest, most lethal fighter is the F-22. The Air Force designed the F-22 as an “integrated” system, which means you can’t pull out an outdated system and plug in a new one. Thus, the F-22 is flying with 1980s computers on board. The Navy uses “federated” systems, which provides some flexibility for “plug and play” capabilities. Why didn’t the Air Force use that approach for the F-22, which will be with us for decades?

And then there’s the Navy’s “littoral combat ship,” or LCS, widely known as the “little crappy ship” because of its inability to survive in the kind of combat it’s supposedly designed to perform.

Part of the problem is the Pentagon and the contractors. The Defense Department always wants to buy ships, aircraft, and missiles faster and more cheaply. To meet the Pentagon’s demands the contractors always respond that they’ll build them faster and cheaper no matter how much it costs you or how long it takes. Thus the continuing massive problems with the F-35 and the USS Gerald Ford.

Dr. Will Roper, the Air Force’s chief acquisition executive, wants to build new combat aircraft in about five years, emulating the practice of the 1960s. But Mr. Roper wants to achieve this goal by sprinkling magic fairy dust on future weapon system acquisitions in the form of more computerized design and manufacturing. It won’t work. The F-35 proved that computer modeling doesn’t forecast accurately how an aircraft will perform once it’s built.

The answer isn’t in Congress’s ongoing attempt to force Mr. Esper to produce the 30-year Navy shipbuilding program. His dissatisfaction with it is rumored to be that the Navy’s plan doesn’t envision sufficient minimally manned or unmanned systems.

Esper also should be, and likely is, unhappy with the Navy’s continued reliance on long-deck carriers, such as the USS Gerald Ford, which have been one of America’s principal power-projection tools since World War II.

Our combat aircraft have to be dominant over any modern battlefield, and if there aren’t friendly airfields for them to operate from, we have to provide our own. Hence the carrier. But it’s reasonable to ask whether the era of the long-deck carrier is about over. A ship that’s a thousand feet long is easy to track by satellite and can be sunk by a non-nuclear Chinese or Russian hypersonic missile. Smaller, cheaper ships that can be produced in far less time — and will be far less visible — than a 100,000-ton carrier may be part of the answer.

Messrs. Esper and Roper are both on the right track, but neither, like anyone else, can tell us what we will need to deter or defeat the threats our nation will face in three decades.

In the movie The Patriot, Mel Gibson tells his sons to “aim small, miss small.” That wisdom should be translated into better weapon system choices. That will require us to design future weapon systems to be smaller, cheaper, and more adaptable to new weapons, sensors, and other subsystems to meet new threats as they evolve.

That is exceptionally hard to do because none of the parties to the acquisition process — the Pentagon, Congress, and the contractors — are structured to function that way. They are used to taking 20 years to design and build new major weapon systems. That number needs to be cut at least in half.

Overhauling weapon system acquisition is a frustrating, thankless job. The last time it was tried was a result of the Packard Commission’s report in the mid-1980s. Despite the obstacles Congress, industry, and the Pentagon threw in its path, the results were pretty good. A modern overhaul like that one is badly overdue, but it’s on no one’s list of priorities.

That’s bad for our national security, which requires weapon systems to be built and fielded in vastly less time than it takes these days.