The P-8 Poseidon multi-mission aircraft, a militarized version of the Boeing 737 jetliner, conducts anti-submarine, anti-surface and shipping interdiction missions from land bases, flying much further with greater payload than any carrier-based plane could.

Poseidon rotates frequently to the bases of overseas allies as it conducts maritime surveillance of the Northern Atlantic, Western Pacific and other areas. The plane uses acoustic sensors and radar to detect hostile warships, and carries munitions such as torpedoes that would be used in wartime to destroy such threats.

However, the Navy says it has a validated warfighting requirement of 138 Poseidons to cope with Russian and Chinese naval forces, and it looks likely to stop purchases at 117—far short of that requirement. Congressional appropriators are asking the Navy how it would replace the aging sub hunters in two Navy Reserve squadrons—which must retire in the near future—if production of the P-8 is prematurely terminated.

During the Cold War, the Navy operated three dozen squadrons of land-based sub hunters as it sought to counter the threat posed by hundreds of Soviet submarines and the powerful surface fleet of the Red Navy. Today it has only a dozen active-duty squadrons, plus those two reserve squadrons—even though the Russians continue to invest in new undersea technology and the Chinese Navy is building out its own submarine force.

The Russian undersea fleet is smaller today than it was during the Cold War, but its boats are becoming more capable—more survivable, more lethal—and Moscow is signaling that submarines will remain a priority in its military preparations. As David Axe, defense editor of The National Interest, observes, “Where the Russian navy is all but abandoning the production of new aircraft carriers, cruisers and other ’blue water’ surface warships, it has recommitted to sustaining a large fleet of big, long-range submarines.”

The newer submarines in the Russian fleet are harder to track than legacy vessels, and some are equipped with anti-ship missiles that can hit adversary vessels hundreds of miles away. Last month, the Russians conducted their biggest undersea military exercise since the Cold War, sending ten subs into the North Atlantic to test the defenses of NATO navies.

China’s undersea fleet is not as capable as Russia’s, but it is improving rapidly. In fact, a recent study by the respected RAND Corporation notes a tenfold gain in Chinese undersea capabilities since U.S. aircraft carriers operating near Taiwan managed to escape detection in 1996. RAND attributes the gain in Chinese capability to quieter subs and the introduction of cruise missiles into the submarine force.

Like Russia, China operates a mixed fleet of both nuclear-powered and diesel-electric subs. The nuclear-powered subs, some of which carry long-range ballistic missiles, have much greater range and endurance than conventionally-powered subs. However, the diesel-electric are thought to be quieter, and thus are well-suited to control chokepoints between China’s marginal seas and the broader Pacific. U.S. naval intelligence fears these subs will increasingly be cued as to the movement of U.S. warships by overhead (i.e., orbital) systems.

The geographical circumstances in which the Russian and Chinese navies operate provide ample opportunity for the deployment of P-8 sub hunters within range of their undersea quarry. But 117 Poseidons will not be sufficient to continuously monitor all areas of interest as hostile submarine forces grow more stealthy and more numerous.

Prime contractor Boeing (a contributor to my think tank) has done a good job of keeping the P-8 on schedule and under budget, leveraging the economies of scale made possible by a global installed base of thousands of 737 commercial transports. The Government Accountability Office has noted the success of the Poseidon’s acquisition strategy, which stressed adapting proven hardware to emerging threats.

India, Australia and the United Kingdom have signed on to the program, and other countries such as Norway and South Korea are signaling interest. Dozens of Cold War maritime patrol aircraft around the world will need to be replaced in the next decade. Whether they are replaced by the P-8, though, will depend on whether the U.S. Navy keeps buying the plane to fill out its warfighting requirement.

Because Poseidon is based on a version of the Boeing 737 that is no longer manufactured for commercial carriers, if production ceases it will likely never resume. Suppliers will stop making unique parts, and money will be too scarce to shift the mission for a handful of additional sub hunters to the next-generation 737 MAX. So the Navy either meets its warfighting requirement now, or it never does. One benefit of doing so is to maximize the interoperability of the U.S. Navy’s land-based surveillance aircraft with the allied fleets which will share anti-submarine and anti-surface missions in the future.

There’s no mystery about why the Navy hasn’t funded additional Poseidon purchases in future years. It is short of money. Despite a sizable increase in funding from the Trump Administration, the service has little hope of ever getting near the fleet size of 355 warships it says it needs to cover the world. Tradeoffs are being made in the purchase of submarines, sealift and other assets vital to countering military moves by Russia and China.

But filling out the Poseidon fleet to the number actually needed by warfighters is not a heavy lift in budgetary terms. The planes cost less than $200 million each, and are much cheaper to keep flying than most military aircraft, thanks to their commonality with commercial 737s. It is very unlikely the Navy will ever get a better deal than it would today by purchasing more P-8s off an assembly line at the peak of its performance.

If the Russian and Chinese undersea fleets keep improving at the pace recently seen, the U.S. Navy is going to need all the Poseidons specified in its warfighting requirement. Stopping now at less than the number the Navy itself says are needed would be a false economy that could lead to operational catastrophe in a future conflict.