U.S. missile defense has focused so much on stopping ballistic missiles from hitting the American homeland, that it has neglected another pressing threat.

Massed salvos of cruise missiles and armed drones could devastate U.S. overseas bases, those vital bastions, such as Guam, that provide airfields, ports and supply bases. While the United States has fretted over the possibility of a few North Korean ballistic missiles hitting the West Coast, China and Russia have amassed huge stockpiles of guided weapons that endanger American bases.

“For most of the post–Cold War era, DoD (Department of Defense) focused its missile defense priorities on fielding ground-based and sea-based kinetic weapons to intercept ballistic threats,” says a new study by the Center for Strategic and Budgetary Assessments in Washington, DC. “With the exception of the Navy, no other Service has fielded major new capabilities to counter cruise missile salvos.”

Titled “Air and Missile Defense At a Crossroads: New Concepts and Technologies to Defend America's Overseas Bases,” the study describes the vast array of threats to American installations. China has more than 1,500 ballistic missiles, including some intermediate-range DF-26 rockets that can reach 2,000 miles deep into the Western Pacific, within range of U.S. bases on Guam. It also has thousands of air-launched cruise missiles, including the CJ-20 with a thousand-mile range. Russia’s land-based Iskander ballistic missile, as well as the air-launched Khinzal hypersonic (Mach 5-plus) weapon, are short-range missiles that could U.S. bases in Europe. Both China and Russia are developing armed drones that can strike distant targets.

While the U.S. Missile Defense Agency is tasked with stopping strategic ballistic missiles, “it is unlikely that the MDA will have the expertise and funding to develop a far more robust system-of-systems with the capacity needed to defend critical infrastructure in the United States and at U.S. military installations overseas against large salvo attacks in the near term,” CSBA says.

That leaves the U.S. Army, which is responsible for providing land-based defenses against theater ballistic missiles and cruise missiles. The Army has 50 Patriot air and missile defense batteries with 480 launchers and more than 1,200 interceptor missiles. However, “they are expensive and their combined capacity would be insufficient to protect airbases and other military infrastructure that U.S. and allied forces would depend on during a major conflict with a great power,” says CSBA.

The Army has just seven batteries, with 42 launchers and 500 interceptors, of Terminal High Altitude Air Defense (THAAD) systems. The Army wants two more batteries, but the $7.5 billion price tag may be too steep. Compounding the problem is a lack of an integrated battle management system for base defense, to coordinate various defensive sensors and weapons.

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CSBA advocates a salvo defense system to protect overseas bases. One possibility is a dual outer and inner defensive ring. The outer ring would consist of satellites and drones equipped with sensors. An integrated battle command system would connect the sensors to manned aircraft and UAVs armed with interceptors to shoot down ballistic missiles during the boost phase, when they are most vulnerable, as well as downing enemy aircraft before they could launch cruise missiles.

The inner layer would be similar, with the addition of drones and ground batteries armed with high-energy lasers, land-based high power microwave weapons whose beams would fry missile electronics, and even Army howitzers firing high velocity projectiles (HVP). “Future artillery may be able to launch HVPs at velocities needed to defeat very fast-moving targets such as supersonic cruise missiles and HGVs [hypersonic glide vehicles].”

This would require developing the requisite drones, lasers, interceptor missiles and microwave weapons. But CSBA argues that base defense should be a priority. “The first step toward achieving this objective is to frame the challenge as a salvo competition between adversaries that each have mature capabilities to attack with hundreds and possibly thousands of guided weapons instead of a small number of ballistic missiles,” CSBA says. “Investing in non-kinetic and kinetic systems effective against the growing threat of cruise missile and UAV attacks would create more robust defenses for the theater bases that DoD continues to rely on for its operations.”

Michael Peck is a contributing writer for the National Interest. He can be found on Twitter and Facebook.