As leaders in Washington face a battle over massive cuts to military spending, they should remember that the outside world continues to turn -- and that Iranian nuclear centrifuges continue to spin.

The $500 billion in indiscriminate defense cuts threatened by "sequestration" -- part of the so-called fiscal cliff that Congress failed to resolve -- could embolden Iran to speed up its nuclear enrichment program, presuming that the United States cannot credibly threaten military force to stop it. This could hasten the inevitable showdown between the U.S. and Iran over the latter's nuclear ambitions, forcing us to answer the difficult question of what we will do if sanctions and threats fail to dissuade Iran from building a nuclear weapon.

Ironically, the sequestration defense cuts would also slow improvements to the very homeland missile defenses that we would need to protect us from an Iranian nuclear missile attack. We've already slashed our missile defense budget by billions of dollars in recent years, ending research and development and canceling improvements to current systems. At less than one-fifth of 1 percent of the defense budget, the cuts to homeland missile defense will hardly make a dent in the federal deficit. But failure to update current systems could someday leave us vulnerable to an Iranian missile attack.

The threat is closer than you think. Iran has already tested intercontinental ballistic missiles by using them to send satellites into space, much like the Soviet Union did with Sputnik. Experts say Tehran may flight-test an ICBM that could reach American shores in just three years or less. It could produce enough enriched uranium for a warhead even sooner than that.

We should be strengthening, not weakening, our missile defenses to stay one step ahead of Iran. That means expanding our homeland missile defense system. Ground-based Midcourse Defense. to an East Coast site, something that Congress is considering. A recent report by the National Academies of Science suggests a new East Coast GMD site would provide a more cost-effective defense against the Iran threat than the Obama administration's Phased Adaptive Approach, which envisions building an entirely new long-range missile defense system in Europe.

Critics argue that expanding missile defense is wasteful because it doesn't work. They simply have their facts wrong. GMD has shot down target ICBMs eight times in realistic tests, and the operational version of the system is three-for-three in testing. Since 2001, U.S. missile defense systems -- including GMD, Aegis, THAAD and Patriot -- have destroyed their targets in 80 percent of tests -- a record once thought unattainable and one that is still improving with technological advances.

Today, prominent Democrats like President Obama and Secretary of State Hillary Clinton endorse the necessity of a strong missile defense as the cheapest insurance against the tragedy of a nuclear attack on a U.S. city. While the U.S. must keep offensive options on the table, they're certainly more costly. Putting boots on the ground in Iran could cost trillions of dollars and untold lives lost. Even efficient airstrike campaigns like the recent one in Libya could cost $2 billion each.

It makes no military or diplomatic sense to weaken our missile defenses just ahead of the most significant nuclear missile showdown since Kennedy, Khrushchev and Castro faced off 50 years ago. Iran's missile treaty with Venezuela portends another Cuban missile crisis.

This belies the fundamental danger of sequestration and the fiscal cliff -- it is budgeting blind, allowing arbitrary dollar figures to dictate military and foreign policy without any regard for reality. That's no way for Washington to address the serious challenges we face, whether confronting Iran's nuclear ambitions or charting a course out of the Great Recession.

Retired Adm. James A. Lyons was commander of the U.S. Pacific Fleet from September 1985 through September 1987.