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

The $500 billion in indiscriminate defense cuts threatened by the sequester — part of the fiscal cliff that Congress did not resolve — could embolden Iran to speed up its nuclear enrichment program, presuming that the U.S. cannot credibly threaten military force to stop them.

This would 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 sequester 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, cutting homeland missile defense will hardly make a dent in the federal deficit. But failing to update current systems could leave us vulnerable to an Iranian missile attack — a threat that is just over the horizon.

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, with enough enriched uranium for a warhead even sooner.

Yet as Iran races to build nuclear weapons, Congress seems paralyzed. Although Republicans and Democrats alike agree that the sequester would be disastrous for our national defense, leading members of both parties now speak as if it were inevitable.

They seem unable to find a compromise that would salvage our best defenses against a nuclear Iran.

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 has told the Pentagon to study. A recent report by the National Academies of Science suggests that 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. But these naysayers sound increasingly out of touch in the face of successful testing. 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 recent tests — a record once thought unobtainable and one that is still improving with technological advances.

While they might disagree on how best to reduce the deficit, congressional leaders in both parties now 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 air strike 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. Venezuela’s Iranian-built missile site is now fully operational and must be removed as it threatens a number of American cities, portending another Cuban missile crisis. This belies the fundamental danger of the sequester and the fiscal cliff — it is budgeting blind, allowing arbitrary dollar figures to dictate military and foreign policy without any regard for reality. It’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.

Adm. James Lyons Jr. is retired from the Navy. He was commander of the U.S. Pacific Fleet from September 1985 through September 1987.