Three years into the Iraq war, facing a spike in casualties from roadside bombings, the Pentagon turned to a steel mill in Coatesville, Pennsylvania, to supply emergency armor for combat vehicles.

The plant had specialized in armor plating for warships and submarines since before the second world war. The only steel left standing after the World Trade Center fell – a wall of forked “trident” columns – was manufactured here.

But in recent years, the Coatesville plant has struggled. At least 150 jobs vacated by retirement and attrition have not been filled, and the plant now operates at just more than half-capacity, workers say. The 580 unionized steelworkers still employed at the site worry that the international conglomerate that owns it, ArcelorMittal, might someday shut it down.

Lately, however, a wave of cautious optimism has crept into Coatesville. In March, Donald Trump announced a new 25% tariff on all US steel imports, and earlier this week, the president ended a temporary exemption from the tariff for the European Union, Canada and Mexico – three close American allies that together account for 40% of imports.

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That news has been greeted with consternation and outrage in La Malbaie, Quebec, where the G7 summit has convened. Theresa May called the tariffs “unjustified and deeply disappointing” in a phone call with Trump last week, Downing Street said. Canada, Europe and Mexico have announced retaliatory levies.

But in places like Coatesville – and elsewhere across the United States where there have been green shoots of activity in the domestic steel industry in recent months – Trump’s tariffs are being welcomed as a long-overdue boost that just might put US steel back on its feet.

Facebook Twitter Pinterest Coatesville, Pennsylvania. Photograph: Mark Makela/The Guardian

Vonie Long, a maintenance tech electrician in the Coatesville plant and the president of the local steelworkers’ union, estimated that about half his colleagues opposed Trump for president and half supported him – a split reflecting regional voting patterns.

But even workers who don’t like Trump approve of the tariffs, said Long, 51. “Overall we’re optimistic that this is going to help us here in Coatesville and as US steelworkers,” he said. “Tariffs on steel are a new ballgame for us. A lot of co-workers are optimistic that it will level the playing field for us.”

Russ Huggins Jr, 53, is a mill truck driver at the plant with 32 years’ experience. He said he voted for Trump and watched many of his Democratic colleagues cross party lines to do the same.

“Nobody helped the steel industry, we were just getting the shaft,” said Huggins. “And so I think that’s a lot of the reason why a lot of the people changed over and did support Trump. I think it’s high time that somebody starts really doing something for the steel industry instead of just talking about it.”

Tom Handley is a maintenance tech millwright at the plant with 12 years’ experience. “It probably might start some crap other places, but for steel it’ll be good,” Handley said of the tariffs. “It’ll be good for us.”

Employment in the US steel industry has dropped by 35% since 1998, and the United States imports nearly four times as much steel as it exports, according to commerce department figures. The Trump administration has declared a goal for the US steel industry to climb to 80% production capacity, up from its current 73%.

Facebook Twitter Pinterest Assorted steelworkers pins are seen in the National Iron and Steel Heritage Museum in Coatesville. Photograph: Mark Makela/The Guardian

But economists warn that steel tariffs could hurt the US economy, raise prices and harm American workers, including steelworkers. The EU has announced $3.3bn in retaliatory tariffs on US imports to take effect next month, and Mexico and Canada have levied similar penalties, raising the specter of a trade war. Even a White House economic analysis reportedly found that Trump’s tariffs would hurt US economic growth.

For Coatesville and places like it, however, there are more pressing problems than the threat of a trade war. Since the decline of big steel in the 1970s and 1980s, the town of about 13,000 people has faced a flat jobs market, shrinking property values and stagnant income levels. Main street is a mix of vacant storefronts and a shuttered barbershop, secondhand stores and a Budget Fuel gas station.

Facebook Twitter Pinterest A bow-tied recliner is displayed for sale on Main Street in Coatesville. Photograph: Mark Makela/The Guardian

“Steelmaking has been part of Coatesville for 200 years,” said Scott Paul, president of the Alliance for American Manufacturing. “The steel mill used to employ more than 5,000 people, so clearly the role in the local economy is somewhat diminished, but it’s still critically important.

“For people in the area who don’t want or need a four-year college degree, it’s one of the few paths that remain to the middle class. It’s a vital part of that economy that’s irreplaceable.”

The Coatesville mill might have been closed completely, locals say, were it not for the intervention of the billionaire investor Wilbur Ross, who led a group that bought the plant in 2003 and later sold it to Luxembourg-based ArcelorMittal.

“A majority of my co-workers, they look at Wilbur Ross as somebody that lifted us up out of the ash,” said Long, the union president.

Ross once again has a key role in Coatesville, as the current commerce secretary and architect of the new tariffs. Ross, whose former firm made a big investment in Chinese steel shortly after he joined the cabinet, declared in a January report arguing for the tariffs that “domestic steel production is essential for national security”.

While Ross was said to have divested his steel holdings to join the cabinet, he maintains close ties to the industry and remained on the board of ArcelorMittal until he took his cabinet post.

Facebook Twitter Pinterest Citizens gather on Main Street in Coatesville. Photograph: Mark Makela/The Guardian

Prime minister Justin Trudeau of Canada, which exported 5.7m tons of steel to the United States in 2017, said the notion of a Canadian threat to US national security left him perplexed.

“One of the things that I have to admit I’m having a lot of trouble getting around is the idea that this entire thing is coming about because the president and the administration have decided that Canada and Canadian steel and aluminum is a national security threat to the United States,” Trudeau said in an interview on NBC.

Ross’s report did not argue that Canada specifically represented a threat to the United States but said that steel imports must be restricted to allow the US industry to grow to a sustainable level.

Trump, however, did not foreclose on the idea that Canada might be dangerous, reportedly telling Trudeau, “Didn’t you guys burn down the White House?” British troops burned the White House in 1814, more than 50 years before the advent of Canada as a self-governing entity.

In any case, the idea that US steel is vital to national security resonates in Coatesville, whose steel plates have been used to build every operational US aircraft carrier as well as nuclear submarines, M1 Abrams tanks and key infrastructure such as bridges and power plants. The plant’s standard products are plates of 160 inches and 206in wide, up to 19ins thick, cut to length.

“One of the advantages that Coatesville has is the fact that it makes armor plate, which is utilized by the military. It’s a substantial part of the business model there,” said Paul. “If the US is building aircraft carriers and other large-ticket items like that, there will be work for mills like Coatesville.”

Facebook Twitter Pinterest A now disused rolling mill at the Lukens Steel Mill in Coatesville. Photograph: Mark Makela/The Guardian

Coatesville is not the only US steel city hoping for a comeback. A blast furnace recently came back online in Granite City, Illinois, creating 300 jobs. A $240m sheet metal production line has been announced in Hickock, Arkansas, and a new rebar micro mill will come to Frostproof, Florida. An idled plant that could employ 1,000 people is scheduled to come back online in Lorain, Ohio.

As chairman emeritus of Bethlehem Steel, Hank Barnette led the acquisition of the Coatesville plant from the Lukens Steel Company in 1997, before Ross bought the combined companies.

“It was and is a premier supplier of plate products really in the world,” Barnette said of Coatesville. “I was always very proud of Lukens.”

Barnette said a comeback for US steel at large is possible, given enough government support over time.

“I think there will always be some imports – but the key is getting the operating rate up year after year after year, for three years, five years, and thereafter,” he said. “It’s not a once-and-done thing.”

The steelworkers agree with the former CEO. “We like doing this work and we think it should be done here,” said Long. “Many of my co-workers share that position: if it could be made here, it should be made here.”