The Standard Missile 3 Block IB, launched by U.S. Navy destroyers, made its combat debut on April 13 when it countered Iran’s barrage attacks on Israel. The SM-3 Block IB intercepted Iranian medium-range ballistic missiles in the exo-atmosphere, more than 62 miles above the Earth’s surface.

Yet just a few weeks earlier, President Joe Biden’s defense budget terminated production of that same missile in an attempt to “pivot from and discontinue” the SM-3 Block IB procurements in favor of the longer-range SM-3 Block IIA. In fact, these missiles are a matched pair with different capabilities needed by the Navy to operate in the harsh regime of exo-atmospheric defense. 

Missile defense has not always been a high priority for the political or defense establishment in Washington. But the war in Ukraine, missile attacks on Israel, and rising threats from China have gotten everyone’s attention. The confusion over SM-3 highlights why the incoming Trump administration must pay closer attention to missile defense production priorities.

The Standard Missile 3 clan specializes in the exo-atmosphere: the cold, dark, upper tier for missile defense and the perfect spot for intercepting and killing ballistic missiles. On Tuesday, another member of the clan, Standard Missile 3 Block IIA, became the first interceptor launched from Guam in a test proving capabilities to defend against potential Chinese attacks. After all, China’s DF-26 missile is chillingly nicknamed the “Guam Express.”

The two missiles, with their confusingly similar names, are not competitors: they are complementary interceptors with different ranges that are designed to counter a wide array of threats from Iran, North Korea, China, and Russia.

Take the example of defending Israel. On April 1, 2024, Iran attacked Israel with a combination of 300 drones and ballistic and cruise missiles. SM-3 missiles were launched from destroyers USS Carney and USS Arleigh Burke in response.

Six months later, on the night of October 1, 2024, Iran launched approximately 200 ballistic missiles at Israel. Ready and waiting were two U.S. Navy destroyers in the eastern Mediterranean: the USS Bulkeley and the USS Cole. Together, the ships launched a dozen missiles to intercept Iran’s barrage. The nose-on shots against incoming missiles proved the SM-3IB’s combat value.

Both variants are needed for these demanding tasks because they are specialized for distinct roles. The SM-3 Block IB has a range of about 700 km. The SM-3 Block IIA has a larger second stage and a range of about 2,500 km. However, while the SM-3 Block IIA has a longer range, it is not a replacement for SM-3 Block IB.

Here's why. The SM-3 Block IIA defends a wider area, which is great for engaging a longer-range, more advanced threat set of medium and intermediate-range ballistic missiles. However, in other scenarios, that might allow the enemy to adjust which missile they launch, or they could try the tactic of depressing their missile trajectories, cutting into the effectiveness of SM-3 Block IIA. In contrast, the SM-3 Block IB, with its shorter minimum engagement range, provides U.S. forces with a complementary, increased engagement window. Missile defense tactics are complex, but this is why the U.S. Navy needs full stocks to deploy both the SM-3 Block IB and the SM-3 Block IIA together. Having both the SM-3 Block 1B and the SM-3 Block IIA missile interceptors available—in launch tubes on Navy ships, for example—gives the warfighter more time and the flexibility to engage with incoming threats with the most appropriate interceptor. By the way, that also improves magazine depth, a significant consideration during combat operations.

Deterring against coercive ballistic missile attacks is more important than ever. For example, a single Russian “Oreshnik” RS-26 intermediate-range ballistic missile with six multiple independently-targetable re-entry vehicles (MIRV) was used against Ukraine on Nov. 21, 2024. Alarmingly, all six MIRV warheads evaded intercept.

The United States must keep the upper hand in the exo-atmospheric fight for its missile defense architecture to be effective. As great as the Patriot and THAAD missile defense systems are, terminal defense alone isn’t enough for the threat landscape. Unconventional new weapons may detonate above terminal interceptor max ceiling heights. They can carry some bad weapons and warheads that need to be intercepted well before they get back into the atmosphere.

With threats looming, the Missile Defense Agency needs both variants of SM-3 in sufficient quantities to make sure the Navy inventory never runs short.

Dr. Rebecca Grant is a national security analyst and vice president, of defense programs for the Lexington Institute, a nonprofit public-policy research organization in Arlington, Virginia. She has held positions at the Pentagon, in the private sector and has led an aerospace and defense consultancy. Follow her on Twitter at @rebeccagrantdc.