Just a few years ago, the beach at Fire Island Pines was almost as wide as a football field, the result of a $1.7-billion government project to combat erosion on the South Shore of Long Island after Hurricane Sandy.

The project seemed to provide a new lease on life to one of New York’s untrammeled gems: a nearly car-free string of beach towns on a 32-mile barrier island that feels like a world apart from the city, and that for decades has provided a wooded haven to both endangered wildlife and members of the L.G.B.T.Q. community.

But today, parts of that same beach are barely as wide as a volleyball net.

Most of that damage was done in the wake of a single winter storm last December that triggered rapid erosion on Fire Island and appeared to undo large parts of that decade-long restoration project in a matter of months.

As the beach succumbs to the advancing ocean, at stake are not just waterfront real estate and legendary beach parties. Fire Island is a crucial part of a string of barrier islands protecting Long Island, one of the most densely populated coastal regions in the United States, from climate change and the extreme weather of the Atlantic hurricane season.

But efforts to protect Fire Island — and the legacy of the government anti-erosion project — appear hamstrung by rules that make the United States Army Corps of Engineers responsible for maintenance and emergency repairs on any beach they have built, a stark change from the more flexible approach that local leaders said was possible before Sandy.

“This is, I think, the worst the beach has ever been in my 25 years on Fire Island,” said Henry Robin, the president of the Fire Island Pines Property Owners’ Association.

“That beach needs to be repaired for reasons that are much greater than my beautiful little summer house,” added Mr. Robin, a retiree who acts as the beach town’s de facto mayor. “This barrier island is the first line of defense for the mainland.”

One Bad Storm

Accessible primarily by ferry from Long Island, Fire Island is home to tiny beach towns that transform into bustling summer colonies, two of which — Fire Island Pines and Cherry Grove — are world-famous L.G.B.T.Q. resorts.

Last December, when the island was virtually empty, a major winter storm wreaked havoc across the United States, whipping up rough seas off Long Island. When the tide went back out, Mr. Robin said, much of the beach went with it.

A spokeswoman for Suffolk County said the South Shore suffered $15.2 million-worth of damage, including the destruction of beach infrastructure — stairs, boardwalks, ramps — that local municipalities repaired. A significant amount of sand was also lost, and the responsibility for replacing it falls on the Army Corps, the spokeswoman said.

After the storm, the county successfully petitioned the Army Corps for emergency repair of the beaches on Fire Island’s western end. A spokesman for the Army Corps, Michael Embrich, said in an email that the cost of “future beach renourishment work” would be divided evenly between “federal and nonfederal funding.”

Repairs on the island’s western beaches are scheduled to begin in the fall, Mr. Robin said. But the Army Corps denied a request to expand the scope of the project to include the island’s eastern side, including Fire Island Pines and Cherry Grove.

“What’s really so incredibly concerning about it is that in previous years, before the Army Corps brought us this beach, we would be fixing it ourselves right now,” said Mr. Robin. “But now we’re at the mercy of the Army Corps.”

Mr. Robin said local leaders were not given a clear reason their appeal for emergency assistance on the eastern side of Fire Island was denied.

Mr. Embrich, the Army Corps spokesman, declined to explain the agency’s reasoning to repair only one section of the island. He said in a statement that engineers “determined that the storms did not meet the federal criteria of extraordinary storms for emergency funding.

“Therefore, according to federal law, Congress must appropriate any additional funds that fall outside the project funding,” Mr. Embrich added. “We will continue to work with our partners at the State of New York to find ways to mitigate storm risk for the residents of Long Island.”

A walk along the beaches of Fire Island, or a quick glance at before-and-after photos that were shared widely online, makes the scale of the problem plain to see.

At low tide, parts of the beach are carved into steep slopes that connect a narrow band of dry sand with the crashing surf. But it is at high tide that Fire Island’s problems become most apparent.

When the tide comes in, beaches that were broad just a few years ago are reduced to spindly, narrow paths. In some places, the water reaches almost to the base of the island’s grassy dunes, and seaweed clumps beneath the steps of the walkways that lead from town to the beach.

Climate change has also raised Fire Island’s water table, Mr. Robin said. That means that as seawater eats away at its edges, the island’s narrow interior takes on more water as well, filling up like a sponge and causing a range of problems.

Parts of Fire Island Pines, including the upscale shops of its tiny business district, have become plagued by a sewage smell on rainy days, as septic tanks do battle with rising groundwater.

And even on sunny days, parts of the town’s main access road, Fire Island Boulevard, flood at high tide, forcing maintenance trucks and emergency vehicles to ford a narrow swamp that was once a road.

“The beach comes and goes.”

Rumaan Alam, a novelist who has spent part of each summer on Fire Island for almost 20 years, said he was shocked to see the state of the beaches when he returned this year.

He said it was “jarring to see what seems like the failure of this big project” as he walked down the beach with his husband and two children from their home in Cherry Grove.

“It is so scary, especially in a season where the Canadian wildfires are obscuring the sky and you’re reading about record heat in the West,” said Mr. Alam. The steep pitch of the sand at low tide can make it treacherous for children and older adults to walk. It has also left beachgoers with few places to sit aside from the dunes, which are fenced off to prevent people from damaging them and further accelerating the pace of erosion.

On the Fourth of July, Mr. Alam saw “a big group of high school girls with their faces painted to say ‘U.S.A.’ just laying out on the dunes,” he said. “There was nowhere else for them to be.”

The erosion also threatened one of the summer’s biggest events: a high-profile fund-raiser called Pines Party that draws hundreds of people to the beach and has raised millions of dollars for L.G.B.T.Q. causes since 1999.

For the first time this year, there was no stretch of beach in Fire Island Pines wide enough to accommodate the all-night event.

Administrators at the Fire Island National Seashore allowed organizers to hold the party on another beach just outside of town, but one day of heavy rain rendered that site unusable, too.

Organizers were forced to relocate the event on one week’s notice, and they only found a new site because a colony of endangered piping plovers migrated away, vacating a new stretch of the national seashore.

Allan Baum, the event’s producer, said the ordeal convinced him to move the party from July to August going forward.

“We won’t have to worry about the birds as much, but we do have to worry about the beach,” said Mr. Baum. “The beach comes and goes.”

An Army Corps Beach

Protecting Long Island from erosion has been on the minds of federal, state and local officials for decades. But efforts to coordinate a response happened in fits and starts, and they were completely suspended for more than a decade in the 1980s and 1990s.

Before Sandy, property owners on Fire Island paid for their own anti-erosion efforts using local tax money every few years, said Mr. Robin.

But that system was no match for Hurricane Sandy, which caused tens of billions of dollars of damage in the New York metro area and killed at least 72 people in the United States, including 65 in the New York, New Jersey and Connecticut area.

After Sandy, the federal government began the $1.7-billion Fire Island Inlet to Montauk Point project to protect 83 miles of Long Island’s South Shore, which makes up roughly 70 percent of Long Island’s shoreline. The project was fully paid for by the federal government, said a spokesman for the Army Corps.

The project’s work on Fire Island included the placement of 2.3 million cubic yards of sand. When it was completed, the island was left with broad beaches, a protective shield of high dunes, and the hope that maybe the problem had been solved.

“The water used to come all the way up to the yards of the houses right on the ocean, and that doesn’t happen now,” said Isaac Namdar, who has vacationed on Fire Island for two decades. “But who knows how long that will last.”

But because the Army Corps is now in charge of the beach, local residents can no longer just organize repairs themselves. Instead, they must work with the federal government, Mr. Robin said.

There are at least three bureaucratic avenues they can pursue, but the Army Corps has already turned down what may have been the quickest and easiest one, he said. Each of the remaining options involves a complex stand-alone project that could take months or years.

Meanwhile, the sea draws ever closer.

“Because of the damage to the beach, we may now be in the most vulnerable state we have ever been in,” said Mr. Robin, looking out over the waves on a recent day. “There’s no beach out there.”