Like its three predecessors, the 2018 Nuclear Posture Review reaffirmed the need for the nuclear triad of bombers, submarine-launched ballistic missiles and intercontinental ballistic missiles. Now comes the hard part.

With the authorization and appropriation cycle for fiscal 2020 now underway, the United States is moving closer to the coming bow wave of modernization efforts necessary to recapitalize it. During the post-Cold War period, when the U.S. faced few real challenges to its military superiority, it was easy to be lax on conventional and nuclear modernization alike, first while taking the peace dividend and then later while focused on counterterrorism.

Geopolitical rivalry is back, and with it a renewed need to steward nuclear deterrence — what former Secretary of Defense Ash Carter called the bedrock of American national security.

One underappreciated attribute of the triad is the distributed quality provided by land-based ICBMs. The program to replace and modernize the ICBM leg is known as the Ground Based Strategic Deterrent. But GBSD is not just about the missile. The program includes silo refurbishment, ground systems and infrastructure, and nuclear command-and-control improvements that will ensure its viability into the late 21st century.

The GBSD modernization program will enhance penetration of enemy missile defenses, improve cyber protection, ease the sustainment and guidance package update process, improve surveillance of the missile fields, permit rapid re-targeting, and perhaps increase the missiles’ payloads to accommodate advanced delivery systems in the future. The program is challenged, however, by a daunting bow wave of increased modernization costs, competing Air Force priorities and the production of solid-rocket motors.

This suite of improvements is long overdue. Originally designed to last for about 10 years, today’s nearly 50-year-old ICBMs are rapidly nearing the end of their service lives, primarily due to aging of their solid-rocket motors.

In the 1950s, the Air Force got into the ICBM business in short order, pulling together a national team to develop and field liquid-fueled Atlas and Titan missiles in the span of a few years. The development of solid-fuel missiles produced the more reliable and prompt Minuteman family, with two variants in the 1960s and the Minuteman III deployed in 1970. In 1990, there were a total of 1,000 deployed ICBMs, of several types.

Today, some 400 Minuteman III missiles remain, which were first deployed a half century ago but have had their avionics and motors replaced. Since then, a combination of regular testing and aging will result in a shortfall of available ICBMs by the early 2030s. The GBSD program must remain on schedule to prevent that shortfall.

The distributed and hardened characteristics of ICBMs creates their quality as a so-called warhead sink. Under current assumptions, each of at least 400 silos would require two warheads each. Any adversary would have to expend a considerable portion of their strategic nuclear force to disable them all. Raising the threshold for nuclear attack strengthens deterrence.

The ICBM leg served its purpose in the Cold War, but the distributive principle will remain important for the foreseeable future. A nuclear force without ICBMs would have a very small number of aim points: two bomber bases and a small number of submarines operationally deployed. Nuclear bombers have long been off alert, so on any given day could be concentrated rather than dispersed. America’s stealthy nuclear submarines remain the most survivable leg, but the removal of ICBMs would permit adversaries to redirect efforts on anti-submarine warfare.

Broad technological and strategic developments are making the principle of distribution more salient for nuclear and conventional military operations alike. As the National Defense Strategy notes, U.S. military superiority can no longer be taken for granted. The onetime American monopoly on precision guidance and exquisite intelligence, surveillance and reconnaissance is now over, and the military services have begun to adapt accordingly.

Both the Navy’s new concept of Distributed Maritime Operations and the Army’s Multi-Domain Operations grapple with the specter of suppression and overmatch from near peers. Both employ maneuver, mobility and distribution to increase the number of aim points and complicate an enemy’s surveillance and targeting.

Competing both with its service culture and large bills coming due on other platforms, the Air Force’s attention to ICBMs has waxed and waned. Other priorities have included the B-21 bomber, the new tanker, and the F-35.

This squeeze may intensify in the coming years as the annual cost of nuclear modernization begins to rise. But there is no time to lose. Although the defense budget process is far from over, a recent markup by House appropriators cut $118 million from GBSD funding for 2020 — a 20 percent reduction from the budget request. Although the cost for GBSD is substantial, it is lower than the procurement and operational cost for either the nuclear submarines or the bomber.

Another potential obstacle is limitations of the domestic industrial base to build a lot of solid-rocket motors. Assuming things go as planned, the Air Force could next year move onto engineering, manufacturing and development for what is expected to be about 640 multistage missiles.

In the defense bill drafts released this week, the House Armed Services Committee report language once again expressed congressional concerns that rocket motor production could slow the GBSD program. In its next report to Congress on the matter, the Air Force may wish to consider using a team of suppliers or some kind of a national team in order to meet capacity, rather than a single source.

These challenges can and must be overcome. The stabilizing quality of distributed ICBMs remains critical to nuclear deterrence. Four nuclear posture reviews over 25 years have affirmed and reaffirmed the need for the triad. Although operated by the Air Force, the contribution of ICBMs to deterrence is a national asset. Congress has been right to question the program’s cost and the precarious state of domestic solid-rocket motor production, and other ways to mitigate risk. Given the coming cliff in the early 2030s, it is incumbent upon the Air Force and congressional representatives to mitigate further delays with the future of ICBMs and the triad as a whole.

Thomas Karako is a senior fellow in the International Studies Program at the Center for Strategic and International Studies.