It was just after dawn when three U.S. warships and a carrier strike group began their long, tense transit through the Strait of Hormuz.
The strait has become a pressure point as Iran increases the heat of its rhetoric against the United States because of fresh economic sanctions imposed against it.
In December, Iranian officials warned the United States not to return to the Persian Gulf after the carrier USS John Stennis departed.
"You want to be always at the max state of readiness to respond to anything," Capt. Richard McDaniel of the USS Sterett said.
Iran recently threatened to close the Strait of Hormuz, attempting to use its control over the waterway as a trump card in its standoff with the West.
One-fifth of the world's oil supply - 17 million barrels a day in 2011 - passes through the strait, and the United States gets approximately 10 percent of its oil supply - 1.7 million barrels a day - from the strait.
Though Iran is not expected to close the strait, analysts still fear that a closure could double the price of oil thereby erasing any prospect of a U.S. economic recovery and plunging the world into a new Great Recession.
Today, ABC News was in the lead ship - the USS Sterett, a destroyer with dozens of missiles and machine guns manned - as the aircraft carrier USS Abraham Lincoln steamed through the strait.
On the bridge, the concentration was intense. The captain and crew monitored radar; surveillance tracked everything that moves.
The Iranians silently shadowed the carrier group as well.
U.S. surveillance showed Iranian navy vessels, drones and a patrol plane flying overhead. Much of Iran's Navy was concentrated here.
The shipping lane is only two miles wide, so there was very little room to maneuver. The USS Abraham Lincoln's dozens of F-18 fighter jets were on alert today. On Monday, before entering the strait, jets roared off the deck one after another on security and training missions.
Today, U.S. helicopters beamed real-time images back to the ship.
Hours into the crossing, the captain was called to the deck. A small boat - similar to those of Iran's Revolutionary Guard Navy that have been harassing U.S. ships in the last few months - had approached.
"Any surface vessel that you see out here, you're definitely going to pay attention to identify, figure out, what they're doing," McDaniel said.
The crew quickly determined that it was a smuggler who eventually turned away.
Naval commanders say these transits are routine, but they also fear that miscalculations on either side could threaten not only these ships but also close down the waterway and put the world's economy at risk.
ABC News' Martha Raddatz contributed to this story.