If diplomacy should fail and Iran were to get a nuclear bomb, the United States still would have ways to discourage Tehran from using the terrifying weapons.
There are limits, however, on what even the world's sole superpower can do to contain a nuclear-armed Iran and blunt its influence in the volatile Middle East.
U.S. officials insist they are not resigned to a nuclear Iran and are pressing negotiations to prevent it from joining the world's club of nuclear-armed nations. At the same time, the administration and the Pentagon are clearly eager to avoid a military confrontation with Tehran.
So Washington has set in place, but not completed, the building blocks of policies to make certain an Iran armed with atomic weapons does not threaten its neighbors.
Those elements include a newly revised defense shield for Europe, plans for coordinated missile detection and defense systems in the Persian Gulf and deeper defense ties to Gulf Arab states fearful of Iran.
The Pentagon has been quietly building up anti-missile systems in the Gulf region for months, to reassure Arab allies like Bahrain and Qatar, and to signal to Iran that aggression against its neighbors would not go unanswered.
"The department's primary focus continues to be enhancing regional security cooperation with our Middle Eastern partners," Defense Department Undersecretary Michele Flournoy told Congress last week. "This focus not only reassures anxious states in the region, but also sends a clear signal to Iran that pursuit of nuclear weapons will lead to its own isolation and in the end make it less, not more, secure."
Last week, Gen. David Petraeus said additional Patriot 3 anti-missile weapons are being installed in the Gulf area. U.S. and allied naval forces, he said, also are interdicting arms smuggled from Iran to its Islamic allies Hamas in the Gaza Strip and Hezbollah in Lebanon.
The United States also is disrupting Tehran's supply lines moving what Petraeus called "prohibited items," technology linked, directly or indirectly, to its disputed nuclear program.
Meanwhile, U.S. military officials are monitoring carefully the growing range and sophistication of Iranian missiles, the presumed delivery system for any eventual Iranian nuclear warhead. There is growing consternation that these missiles might also be used to deliver conventional weapons against Iran's neighbors.
The Iranian missile arsenal includes mid-range ballistic missiles capable of hitting Arab sates, Israel and central Europe, as well as short-range Iranian missiles that could be used against U.S. forces in Iraq.
The Defense Intelligence Agency recently said that with outside help, Iran could one day develop an intercontinental ballistic missile capable of reaching the United States.
Obama administration officials and military leaders say that as Iran nears the point, perhaps in a year's time, when it could build a bomb, the room for military and diplomatic maneuvering by the United States is shrinking.
President Barack Obama has said Iran cannot be allowed to become a nuclear weapons state. Despite that red line, there is a strong distaste among military leaders and the White House for seeking to resolve the Iranian problem with military force.
Joint Chiefs of Staff Chairman Adm. Mike Mullen and others have not budged from their view that a U.S. or Israeli military strike on Iran's known nuclear development facilities would not prevent Tehran from building a bomb eventually.
Instead, they warn, an attack on Iran's suspected weapons sites could cause a far-reaching and unpredictable backlash.
But U.S. military and diplomatic officials also worry about subtler questions. One is what should the United States do if Iran should develop the full range of technologies, know-how and materials to build a bomb but stop just short of assembling one? What would be the appropriate, proportionate response?
For now, the administration's main focus is to win support in the U.N. Security Council for a new round of economic sanctions, imposed because of Iran's alleged failure to comply with its responsibilities as a signatory of the Nuclear Non-Proliferation Treaty.
These penalties would be a starting point for additional economic and trade penalties imposed by the United States, individual European allies or others.
The Obama administration hopes that by inflicting economic and diplomatic pain, it can persuade Iran to rethink its nuclear ambitions and avoid military action.
But some experts have warned the United States must have a plan for containing a nuclear-armed Iran, if sanctions and other measures fail.
In a statement Sunday, Defense Secretary Robert Gates made clear that the Obama administration is grappling with the question of what to do against Iran's nuclear program short of war.
Gates referred to an Iran memo he wrote in January that identified "next steps in our defense planning process" where further policy decisions would be needed in the weeks and months ahead.
He offered no specifics about what the memo contained, but said it had presented questions and proposals to advance the internal discussions.
Gates submitted the memo after the expiration of Obama's deadline for Iran to accept his offer to hold direct nuclear talks.
Those talks, had Iran agreed to them, would have been a test of Tehran's assertions that it has no intention of building a nuclear bomb.
But even as Gates submitted his memo to the White House, the administration was shifting its focus to gathering international support for new sanctions against Iran.
The sanctions path is far from smooth. China in particular is reluctant to impose harsh new penalties on Iran, from whom it imports a substantial portion of its oil, although in recent days Beijing has agreed to begin discussing possible sanctions.