It was smoke, curling from where the two towers had once stood, back-lit by emergency lighting erected to help the rescue operation. Barely 15 hours before, the Tahoma was patrolling fisheries off the New England coast. Now she was dropping anchor between the Battery and the Statue of Liberty, ready to assume command-and-control functions for dozens of armed Coast Guard vessels. “No matter what happens to you in your life, things can become routine,” says Lt. Sherman Lacey, the Tahoma’s operations officer. “I really doubt anyone is treating this as routine.”

The Tahoma has been parked off Manhattan for three weeks now, and there’s still nothing routine about its mission of protecting a domestic harbor against terrorist attacks. It’s not that the Coast Guard hasn’t protected harbors before. That’s one of its many missions. But Coasties now board every vessel entering New York Harbor. Their boats patrol the rivers, providing security for the United Nations, monuments and bridges. As the only branch of the military with domestic law-enforcement duties, the Coast Guard is at the front lines of homeland defense. In New York, that job is getting its shakedown cruise.

The biggest responsibility is boarding every ship. That happens south of the Verrazano Narrows off Staten Island. The Coast Guard has 20 to 30 marine inspection teams in the harbor, checking every vessel’s crew, passenger and cargo manifests. It’s a heavy workload for a corps already stretched thin by other duties. Coasties do search-and-rescue, fisheries-law enforcement, drug interdiction, environmental protection and vessel-traffic control with only 35,000 active-duty personnel. For this, the largest nationwide port and waterway security operation since World War II, the Coast Guard has activated 2,600 reservists. So far, they’ve stopped and boarded 600 vessels in New York Harbor alone. “Boats are getting tired and people are getting tired,” Cmdr. Gary Smialek, captain of the Tahoma, says over lunch in the wardroom. “We don’t know what the plan is, but there will be a Coast Guard presence here for the foreseeable future.”

That presence includes heavily armed boats skimming the swells around Manhattan. At the Hudson River pier where the emergency operations center is headquartered, raider boats patrol. These are Boston Whalers, 26-foot boats with two 175-horsepower outboard motors, a .50-caliber machine gun at the front and an M-60 machine gun on each side. As we cruised up the river in a Coast Guard Auxiliary boat–actually a sport boat volunteered by its owner–they bobbed on the water, wondering what we’re doing.

Jim Dipelesi, our boat’s captain, picked up a bullhorn. “We’d like you to do a flyby,” he said. “But not too close!” The camouflage-fatigued team on the raider waved and laughed, and buzzed toward us, near enough to spray us with wake water before peeling off. The four crewmembers were young and good looking, from the man behind the wheel to the woman handling the main gun.

If the Coast Guard is to have an increased homeland defense role, these Port Security Units may be on the front lines. They’re designed to deploy to a hotspot anywhere in the world in 96 hours, and have boats on the water 24 hours after that. It’s called “force protection in theater,” and usually it means a long ride in a military cargo plane. Three weeks ago, it meant a bus ride to the Coast Guard base on Staten Island. Usually a PSU handles security in ports overseas; units were called in to help the Navy after the USS Cole was attacked in Yemen. Members face language and cultural differences (as well as prickly local authorities) and eat field rations from vacuum-sealed envelopes. Here, they live in a “tent city,” but eat at the base galley.

I tell the unit’s commander, Robert Grabb, how strange it seems to see gunboats patrolling the harbor. “It’s not as if you’ve got an unpopulated shoreline, so it would be extremely unlikely for us to let loose,” he says. Then again, his unit is trained not necessarily to shoot, but to “interpose” their boats between the object they’re protecting and a threat. The boats are fast enough to get in harm’s way.

From the wooden pathways built to keep the mud down in the tent city, I sneak glances into military-green tents at gear-laden bunks. There’s a “morale tent,” with a donated Grill Master 560, as yet unused, and a TV-satellite dish outside. It has a wooden sign, painted with the dolphin silhouette that is the unit’s logo and the words CAMP NOBLE EAGLE. The sun has come out after two days of rain, and a few Coasties are sitting at a picnic table in civilian clothes, listening to one of them play guitar. It looks a like a good summer camp, until I meet Seaman Samantha Johnson. She’s a reservist, 25, in full camos, and the M-16 slung over her shoulder is entirely uncamplike. In civilian life, she’s a police officer in Virginia Beach, Va. “When we’re training, we’re thinking about going overseas,” Johnson says. “Now we have to think about places with bridges and buildings, places in our own country we’re going to have to secure … That’s what I think is scary. You look at other countries and see soldiers on the street, and now that’s how our home feels.”

The Coast Guard probably can’t sustain this level of security forever. Three weeks ago there were about 500 Coast Guard personnel working in the New York area; now there are 2,000, which means lots of other jobs–like the Tahoma’s fishery patrols–aren’t getting done. The PSU raiders are open-canopied and made of fiberglass, unfit for use in icy winter waters. All those reservists have to go home some time. “Right now, after the attack on the trade center, all bets are off,” says Lieutenant Lacey. We’re on the bridge of the Tahoma, looking at Manhattan’s diminished skyline. “I don’t think anybody can say that something isn’t imaginable.” The question is, who will be there to interpose themselves between us and whatever happens next?