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The United States has quietly moved significant military reinforcements into the Gulf to deter the Iranian military from any future attempt to shut the Strait of Hormuz and to increase the number of fighter jets able to strike deep into Iran if the standoff over its nuclear program escalates.
The effort is part of a long-planned effort to bolster the U.S. military presence in the Gulf, in part to reassure Israel that, as a senior administration official put it last week, "when the President says there are other options on the table beyond negotiations, he means it."
But at a moment that the United States and its allies are beginning to enforce a much broader embargo on Iran's oil exports, meant to force Tehran to take seriously negotiations over sharply limiting its nuclear program, the buildup carries significant risks, including that Iran's powerful Revolutionary Guard could decide to lash out against the increased presence.
The most visible elements of this buildup are ships designed to enhance vastly the ability to patrol the Strait of Hormuz and to reopen it should Iran attempt to mine it to prevent Saudi Arabia and other oil exporters from sending their tankers through.
The Navy has doubled the number of minesweepers assigned to the region, to eight vessels, in what military officers describe as a purely defensive move.
"The message to Iran is, 'Don't even think about it,"' said a senior Defense Department official. "Don't even think about closing the strait. We'll clear the mines. Don't even think about sending your fast boats out to harass our vessels or commercial shipping. We'll put them on the bottom of the Gulf."
Over recent weeks, additional F-22 and F/A-18 warplanes have moved into two separate bases in the Gulf to bolster the combat jets already in the region and the carrier strike groups that are on constant tours. And the Navy, after a crash development program, has moved a converted amphibious transport and docking ship, the Ponce, into the Gulf to serve as the Pentagon's first floating staging base.
The initial assignment for the Ponce, Pentagon officials say, is to serve as a logistics and operations hub for clearing mines. But with a medical suite and helicopter deck -- and bunks for combat troops -- the Ponce could eventually be used as a base for Special Operations forces to conduct a range of missions, including reconnaissance and counterterrorism, all from international waters.
For President Barack Obama, the combination of negotiations, new sanctions aimed at Iran's oil revenue and increased military pressure is the latest test, and perhaps the most vital one, of what the White House calls a two-track policy against Iran. In the midst of a presidential election campaign in which his presumed opponent, Mitt Romney, has accused Mr. Obama of being "weak" in dealing with the Iranian nuclear issue, his goal is to project toughness without tipping into a crisis in the Gulf.
At the same time he must signal support for Israel -- but not so much support that the Israelis see the buildup as an opportunity to strike Iranian nuclear facilities, an attack that Mr. Obama's team believes could trigger a war without significantly setting back the Iranian program.
A key motivation for the covert effort to undermine Iran's enrichment capability with cyberattacks has been to demonstrate to the Israelis that there are more effective ways to slow the program than to strike from the air.
So far, there is little evidence that the increased pressure is having the desired effect on Iran. Negotiations are in stalemate, though a group of Iranian, American and European experts are expected to meet in Istanbul on Tuesday to review a recent U.S. proposal and Iranian response. Iran has strenuously resisted all efforts to force it to give up enrichment of uranium.
U.S. Defense Department officials emphasized that the recent reshaping of forces in the Gulf region should not be viewed as solely about the potential nuclear threat from Iran.
"This is not only about Iranian nuclear ambitions, but about Iran's regional hegemonic ambitions," the senior Defense Department official said. "This is a complex array of American military power that is tangible proof to all of our allies and partners and friends that even as the U.S. pivots toward Asia, we remain vigilant across the Middle East."
While all U.S. ground troops have been withdrawn from Iraq, a force equivalent to an extra Army combat brigade has remained in Kuwait, officials said. It could have many roles to contain regional instability, but Iran is a primary concern.
"There's significant risk that as pressure builds, the Iranian response could be to lash out," a military official central to the effort said in the spring. "The buildup is designed to reduce that risk. The Iranians are usually pretty rational actors, but there is no guarantee."
While it always is difficult to read Tehran's intentions, senior Navy officers have noted that Iranian ships in the Gulf have recently refrained from provocative behavior.
"Things have been, relatively speaking, quiet," said Adm. Jonathan W. Greenert, the chief of naval operations, assessing actions by Iranian Navy vessels over "the last couple of months."
But that was without the pressure of the new sanctions; Iran is already exporting far less oil every day than a year ago, about 1.5 million barrels a day versus 2.5 million before the gradual imposition of earlier sanctions.
While Iranian vessels have avoided any confrontations with allied warships in recent weeks, Iran expects to equip its ships in the Strait of Hormuz soon with shorter-range missiles, an Iranian Revolutionary Guard commander said Friday. The U.S. attack aircraft recently moved to the region give the United States military additional capability against those missile batteries.
Ali Fadavi, the commander, said Iran had already equipped its vessels in the Gulf with missiles that could strike targets at distances up to 220 kilometers, or 135 miles, and was expecting to introduce new missiles with a range of 300 kilometers soon, according to the semiofficial news agency Mehr.