America's nuclear arsenal is maintained and modernised by three national laboratories under the National Nuclear Security Administration (NNSA): Lawrence Livermore National Laboratory near San Francisco, Los Alamos National Laboratory in New Mexico, and Sandia National Laboratories (also in New Mexico). Livermore and Los Alamos design the "physics packages"—the nuclear components of warheads—while Sandia adds non-nuclear components (triggers, batteries, sensors, radiation-hardened electronics) and integrates the devices with delivery systems such as missiles. The labs employ tens of thousands of scientists and engineers.
Los Alamos built the first nuclear bombs during the Manhattan Project. Livermore was established in 1952 as a deliberate rival, to bring "scientific rigour, peer review and competition" to weapons design; the two labs pursue different designs and refer to each other as "competimates". One of Livermore's early tasks was miniaturising thermonuclear bombs for submarine-launched missiles; its Polaris missile system, developed in under four years, was an order of magnitude smaller than anything before it and is considered one of the most important advances in the history of nuclear weapons.
Unlike the fission bombs of the Manhattan Project, which split atoms of uranium and plutonium, thermonuclear (hydrogen) bombs have two stages: a plutonium fission bomb creates an intense burst of heat that ignites a second stage of deuterium-tritium fusion. This technology enabled more powerful yet more compact weapons that became the workhorse of both American and Soviet arsenals during the cold war.
America stopped underground explosive testing in the early 1990s. To compensate, the labs developed new tools to guarantee the safety and reliability of ageing warheads. Every year a few devices are taken apart and examined. The National Ignition Facility (NIF) at Livermore, the world's most powerful laser, fires 192 beams totalling some 500trn watts at microscopic material samples to simulate nuclear-blast conditions. Sandia's Z machine approximates a nuclear blast using intense electromagnetic fields. At Los Alamos, non-nuclear weapon components are tested with conventional-explosive shock waves. In December 2022 NIF became the first facility in the world to achieve fusion ignition—releasing more energy from fusion than was used to initiate it. By mid-2025 it had achieved ignition on nine occasions, gradually increasing the energy yield each time.
The Advanced Simulation and Computing (ASC) programme, launched in 1995 alongside NIF, builds the supercomputers that simulate nuclear explosions in three dimensions. In January 2025 the NNSA unveiled El Capitan at Livermore, the world's most powerful supercomputer. El Capitan can perform a quintillion (10¹⁸) floating-point operations per second—roughly 100m times faster than a typical laptop—making it only the third exascale computer ever built. It uses custom accelerated-processing units (APUs) developed by Advanced Micro Devices, in which CPU and GPU chiplets share a single piece of silicon, eliminating the memory-bus bottleneck. Simulations that once took months can now run more than 200 times a day at much higher resolution.
The W93, the first new warhead in the American nuclear arsenal since the 1980s, is being designed at Los Alamos for the US Navy's Columbia-class ballistic-missile submarines. With explosive testing off-limits, the entire design process relies on simulation from the start. The NNSA's budget has roughly doubled in the past five to six years, funding thousands of additional scientists, modernised facilities and a restored capacity to manufacture plutonium pits, a core element of thermonuclear bombs. Unlike many other areas of American science, the NNSA expects no cuts in federal funding.
America's ICBM infrastructure comprises 400 missiles deployed in 450 silos across the great plains, connected by a spider's web of cables to 45 missile-alert facilities (MAFs). Each MAF consists of a peanut-shaped capsule below ground and a support building "topside" above. The 90th Missile Wing is based at the F.E. Warren air-force base in Cheyenne, Wyoming. Launching an ICBM requires decrypting and verifying orders, receiving unlock codes and then two officers per capsule turning keys and levers simultaneously; at least two separate capsules must act in unison.
The ageing Minuteman III ICBMs are to be replaced with the Sentinel missile. In 2024 the estimated cost of Sentinel jumped to $141bn, more than 80% above the previous projection. The overrun stems not from the missile itself but from supporting infrastructure dating to the 1960s and 1970s: weakening cement and water infiltration mean it would be "cheaper and faster to just dig a new silo" than to refurbish old ones. The original plan envisioned reusing existing facilities after a light refurbishment. Replacing old copper cables with fibre-optic ones would allow more data to flow and reduce the number of MAFs from 45 to 24. Sentinel was supposed to begin entering service in 2030 but may not do so until 2038; the air force is having to extend the life of the Minuteman III in the meantime. Sentinel is expected to remain in service until the 2070s.
Cold-war programmes explored making ICBMs mobile. The Midgetman was a small ICBM carried on a road-mobile launcher; the Peacekeeper railway garrison carried a large missile on special railway carriages. Both were intended to be dispersed across America's transport network in times of crisis but were abandoned with the end of the cold war. A congressionally appointed bipartisan commission in 2023 recommended examining mobile options anew, but the air force judged it too expensive and unpopular.
Critics such as Daryl Kimball of the Arms Control Association argue that ICBMs are destabilising because their fixed, known locations give a president only minutes to decide whether to launch or lose them, and that America should rely instead on a "dyad" of submarine- and air-launched weapons. Proponents counter that without ICBMs, an enemy could attempt to decapitate America's deterrent by striking the handful of bomber and submarine bases; with 450 silos, an adversary must fire hundreds of nuclear missiles at the American heartland, which would undoubtedly be detected and invite massive retaliation. China has built hundreds of silos in recent years, apparently accepting this logic.
The last treaty limiting American and Russian nuclear weapons, New START, expires in February 2026, with no replacement in sight. It capped each side at 1,550 "strategic" (long-range) warheads. Once it expires, Congress has earmarked money to "upload" stored warheads onto existing bombers, ICBMs and submarines. Some experts urge both sides to continue observing the ceiling informally pending new negotiations; others see uploading as the likely next step in the arms race.
Thom Mason, director of Los Alamos, describes the current era as the "fourth age" of nuclear weapons: the first was the Manhattan Project; the second was the cold-war arms race; the third was the post-Soviet period in which nuclear deterrence seemed to be declining in importance. The fourth age is characterised by the breakdown of arms control, Russia's threats of nuclear use, China's rapid build-up, tensions among nuclear powers such as India and Pakistan, uncertainty over new and would-be nuclear states, and the risk that American allies may develop their own weapons as they lose faith in the American nuclear umbrella.
Lord, defend me from my friends; I can account for my enemies.