The buildup is largely—though not entirely—defensive and suggests the United States is not preparing an attack but rather leaving the initiative to Iran and deploying the means to foil any Iranian initiative.
A key part of the new deployments are mine countermeasures—
Mines were the main tactic Iran used in its confrontation with the United States in the Persian Gulf in the 1980s. And, since then, the Islamic Republic has vastly boosted its mine-laying capability, suggesting it still sees mines as its best route for causing trouble for the United States.
The new reinforcements – coming amid faltering negotiations over Iran’s nuclear program and escalating tensions over Syria – include 
US officials are casting the moves as deterrent in nature, designed to discourage any attempts at closing the Strait of Hormuz.
“The message to Iran is, ‘Don’t even think about it,’” an unnamed senior Defense Department official told The New York Times. “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.”
Also added are contingents of small underwater drones that can detect and detonate underwater bombs. The US ordered dozens of these drones, known as SeaFox, in February, and the first arrived in 
The US Navy is taking mine-clearing operations seriously because of a range of underwater threats, including Iranian mini-submarines that are “a huge problem” for the United States, according to retired Navy Cmdr. Christopher Harmer, who was the director of future operations for the US Navy Fifth Fleet in the Persian Gulf from 2008 to 2009.
“They are a huge problem for us because they can disperse them throughout the Persian Gulf and the Arabian Sea, and it’s extremely difficult for us to track them,” he said. Those mines could then target US Navy ships as well as commercial lines, making it impossible to ship cargo – including oil – through the strait.
The US does best detecting larger, Soviet-made submarines, but the mini-subs that the Iranian navy has been developing and deploying over the past few years are more difficult to detect because they operate in shallower waters where sonar capabilities are not optimal. These mini-submarines could quickly deploy mines and also lay in “wait to execute an ambush,” according to Harmer.
The US this month also deployed the USS Ponce, a converted transport and docking ship, which will serve as a logistics and operational hub for mine-clearing operations, basically a floating base.
The Ponce is also equipped with several hundred bunk beds, a medical suite and a helicopter deck – capabilities that enable it to function in the future as a platform for humanitarian aid, reconnaissance missions and hostage rescues—not to mention covert operations against Iran.
The remodeled Ponce, anchored in international waters, could stage independent off-shore operations without needing permission from a host country such as is required for operations from bases in the Arab states.
The versatility of the Ponce, billed as an “afloat forward operating base,” has inspired the US Navy to ask Congress for $1.2 billion to build two new such ships. If approved, the first ship on this wish list would be ready in 2015. That order for a ship designed specifically to deal with Iran means the US Navy is planning for the long haul and not expecting the current tussle to be over in the short term.
In another move, the US is sending the USS John Stennis – an aircraft carrier – to the Persian Gulf four months ahead of schedule to shorten the time a single carrier will be in the region.
The Stennis is supposed to join the USS Dwight Eisenhower, which could have been left alone for several months had the deployment of the Stennis not been speeded up. The new orders mean the Stennis has to accelerate its re-equipping and crew training because it just left the Persian Gulf in December.
“This is not only about Iranian nuclear ambitions, but about Iran’s regional hegemonic ambitions,” said the unnamed senior Defense Department official.
Canada has also been maintaining a presence in the region, joining forces with the US. It has sent the HMCS Regina to replace the Charlottetown, which has been stationed in the Persian Gulf since it took part in the NATO mission off Libya.
The Regina is expected to remain in the Persian Gulf until December or January, at which time another Canadian ship will replace it.
“We are probably going to continue the cycle for some time,” a senior Canadian military officer told Canada’s Postmedia News, as a sign of the country’s plans, announced last year, to establish a permanent presence in high-stakes waters of the world, including the Persian Gulf.
While the Islamic Republic complains most loudly about the US presence in the region, Canada, Britain and France all now have permanent deployments around the Persian Gulf, while other countries like Australia and several NATO states send ships into the Persian Gulf periodically. The US and 19 other countries will also be conducting joint exercises in the coming months to improve their mine countermeasures in the waterway.
The United States Navy has had a permanent presence inside the Persian Gulf since 1948. The size of that presence was increased after the Islamic revolution of 1979, but the presence itself is due to oil, not frictions with Tehran.
The Arab states around the Persian Gulf are taking steps to boost their own capabilities, including buying American-made air defense systems and other weaponry.
While there has been no new deployment of US ground forces, about 13,000 troops will remain in Kuwait despite the total American withdrawal from Iraq—far more ground troops than the US military has ever before maintained in the region in peacetime.
The US Defense Department this month issued an unclassified assessment of Iran’s military power, noting improvements in the range and lethality of the Islamic Republic’s missiles. Unlike previous such reports, this assessment did not downplay Iran’s ability to defend itself, including its asymmetric warfare capability to mobilize proxies in the region.
“The report seemed pretty sober and respectful of Iran’s capabilities, crediting Iran with improving survivability,” said Kenneth Katzman, an analyst with the Congressional Research Service.
The report says that Iran has improved the accuracy of its missiles. That is crucial. Iranian missiles have largely been dismissed over the years as militarily useless because they are so inaccurate. Iran likes to boast that it can hit 35 US bases in the region. Military specialists say US bases are within range of Iran, but that Iran would have to fire fusillades to have any hope of actually hitting a high value target. Iran has said it has been working on improving accuracy—and the new Pentagon report says it has improved missile accuracy. But the report does not say how much the accuracy has been improved and weather the improvements are enough to be militarily significant.