More than a decade after the United States withdrew from the Anti-Ballistic Missile Treaty and began fielding an initial missile defense capability, the U.S. missile defense program is but a shadow of the robust program needed to protect the nation.

Because the United States and its allies face numerous missile threats from multiple potential adversaries, the George W. Bush administration pursued a layered defense against missiles of all ranges in all phases of flight. This included deployment of a modest number of anti-missile interceptors in Alaska and California, primarily to counter North Korean launches against the U.S. homeland.

This Ground-Based Midcourse Defense system was to be a building block for more robust capabilities. As the White House noted in 2003, “These capabilities will serve as a starting point for fielding improved and expanded missile defense capabilities later,” which could include “a family of boost-phase and midcourse hit-to-kill interceptors based on sea-, air-, and ground-based platforms” and “development and testing of space-based defenses.”

Since then, although potentially hostile states have expanded their missile capabilities, U.S. missile defense programs have been cut, with promising technologies curtailed or eliminated and funding priorities driven by a quest for elusive budget efficiencies that subordinate long-term strategy to short-term fiscal considerations. Moreover, the effectiveness of the GMD system has been questioned due to several failed intercepts — including the last three consecutive tests — resulting in an overall success rate of only 50 percent.

Consequently, the United States is banking on a single missile defense system deployed in limited numbers and of uncertain reliability to protect the entire country from the evolving threat. Defending the nation from missile attack is technically challenging, and exclusive reliance on a single system is neither wise nor prudent. The United States clearly needs what some in Congress have called a “hedge.”

U.S. nuclear forces have long been based on a triad of land, sea and air platforms to ensure the robustness of the strategic nuclear deterrent. Like a diversified stock portfolio that minimizes risk by spreading assets, the triad is a hedge against the technical failure of, or the ability of an opponent to counter, one or more legs. It serves the nation well and, despite occasional calls to abandon it, every administration — including the current one — has reiterated support for the triad because of the diversified approach it embodies.

Similarly, our missile defense posture should be one of “defensive diversity,” where we don’t put all our interceptor eggs in one basket. To ensure robustness, the GMD system should be augmented with other systems capable of defending the U.S. homeland.

The Aegis-based Standard Missile-3 offers such a potential and has a superb test record. Since 2002, the SM-3 has scored 26 intercepts in 32 tests, an impressive 81 percent success rate. The missile’s Block IA version is effective against a range of short- to intermediate-range missile threats, while the latest version — the Block IB — achieved its fifth consecutive successful intercept earlier this month. With adequate sensor data, the SM-3 can be upgraded to counter missile threats of even greater range and sophistication.

When deciding on future missile defense spending, Congress must remember that the United States is not vulnerable solely to long-range intercontinental ballistic missiles. Shorter-range missiles launched from offshore pose an equally dangerous threat. A high-altitude nuclear detonation from a single offshore missile launch can cause a devastating electromagnetic pulse effect that could cripple American society. Augmenting the GMD system with a sea-based platform to counter these types of missile threats should be a no-brainer.

In this period of sequestration and budget austerity, some in Congress will argue that adding a sea-based defensive capability to our existing ground-based interceptors is costly, redundant and unnecessary. But neither authorizers nor appropriators should confuse cost with value. The value of an additional defensive capability against ballistic missile attack — one with an excellent test track record — is incalculable.

Missile defense is not a “one size fits all” proposition. To lower risk and heighten confidence, a policy of defensive diversity makes sense. This means reconsidering missile defense from space to provide global protection from ballistic missiles in multiple stages of flight and keep pace with evolving missile threats.

The current situation is analogous to the pre-9/11 world, when the United States found itself unprepared to deal with hijacked aircraft used as weapons of mass destruction. The 9/11 commission called this a “failure of imagination.” It should not take an actual attack by missiles carrying weapons of mass destruction before our leaders in the White House and on Capitol Hill decide to provide Americans with the protection they deserve.

David J. Trachtenberg, president and CEO of Shortwaver Consulting, is a former principal deputy assistant secretary of Defense for international security policy and staff member on the House Armed Services Committee.