“Their surface fleet would be at the bottom of the ocean, but they could score a lucky hit,” said Michael Connell, the director of the Iranian studies program at the Center for Naval Analysis, a research organization for the Navy and Marine Corps. “An antiship cruise missile could disable a carrier.”

Iran has two navies: one, its traditional state navy of aging big ships dating from the era of the shah, and the other a politically favored Revolutionary Guards navy of fast-attack speedboats and guerrilla tactics. Senior American naval officers say that the Iranian state navy is for the most part professional and predictable, but that the Revolutionary Guards navy, which has responsibility for the operations in the Persian Gulf, is not.

“You get cowboys who do their own thing,” Mr. Connell said. One officer with experience at the Navy’s Fifth Fleet headquarters in Bahrain said the Revolutionary Guards navy shows “a high probability for buffoonery.”

The Revolutionary Guards navy has been steadily building and buying faster missile boats and stockpiling what American experts say are at least 2,000 naval mines. Many are relatively primitive, about the size of an American garbage can, and easy to slip into the water. “Iran’s credible mining threat can be an effective deterrent to potential enemy forces,” an unclassified report by the Office of Naval Intelligence, the American Navy’s intelligence arm, concluded in 2009. “The Strait of Hormuz is a narrow chokepoint that could be mined effectively in a relatively short amount of time” — with disruptions within hours and more serious blockage in place over days.

Although the United States would respond with minesweepers, analysts said American naval forces might encounter layers of simultaneous attacks. The Iranians could launch antiship missiles from their coastline, islands or oil platforms and at the same time surround any American ship with missile-armed speedboats. “The immediate issue is to get the mines,” Mr. Connell said. “But they’re going to have to deal with the antiship cruise missiles and you’ll have small boats swarming and it’s all going to be happening at the same time.”

The United States could take out the antiship missile launchers with strikes from fighter jets or missiles, but analysts said it could take time to do so because the launchers on shore are mobile and often camouflaged.

The tight squeeze of the strait, which is less than 35 miles wide at its narrowest point, offers little maneuvering room for warships. “It would be like a knife fight in a phone booth,” said a senior Navy officer. The strait’s shipping lanes are even narrower: both the inbound and outbound lanes are two miles wide, with only a two-mile-wide stretch separating them.