U.S. Steel and Nippon Steel announced on Friday that they had entered into an agreement with the U.S. government to seal the terms of a “partnership” between the companies, more than a year after the Japanese steel maker first tried to buy its U.S. competitor.

The agreement paves the way for the two companies to finally close a deal that has been in limbo for a year and a half. Nippon Steel’s 2023 bid for U.S. Steel became ensnared in presidential politics and inflamed tensions between the United States and Japan, a close ally and trading partner. But the Trump administration decided that with sufficient federal government control it was worth giving Nippon Steel the opportunity to invest in a 124-year-old American manufacturer that has declined in recent decades.

President Joseph R. Biden Jr. had signed an executive order blocking the acquisition in early January, saying its security risks could not be resolved by a national security agreement. On Friday, President Trump issued an executive order concluding that such an agreement could, indeed, resolve the national security concerns.

The companies then announced that they had signed that agreement, though details of its contents were unclear.

“Today’s executive order ensures U.S. Steel will remain in the great commonwealth of Pennsylvania, and be safeguarded as a critical element of America’s national and economic security,” Kush Desai, a White House spokesman, said in a statement.

The companies referred to the deal as a partnership, echoing language that Mr. Trump used in describing the transaction, which he blessed three weeks ago. But U.S. Steel has not indicated to shareholders that it has altered the $14.9 billion sale to Nippon that they approved 14 months ago. Leaders of the United Steelworkers union, which initially protested the deal, have called out significant similarities between the acquisition that Nippon Steel originally proposed and the partnership that Mr. Trump has described.

The companies issued a statement thanking “President Trump and his administration for their bold leadership and strong support for our historic partnership.”

“This partnership will bring a massive investment that will support our communities and families for generations to come,” the statement said.

A spokesman for the United Steelworkers said the union did not yet have a comment because it had not seen the terms of the companies’ security agreement.

U.S. Steel said in a statement that the agreement called for roughly $11 billion in new investments by 2028. The deal will also give the U.S. government a “golden share” in the company, a rarely used practice through which the government takes a stake in a business. In the United States, the government has typically taken a stake only in companies that are ailing or in particular need of government attention, like General Motors during the 2008 financial crisis.

“We have a golden stock. We have a golden share, which I control, or the president controls,” Mr. Trump told reporters on Thursday. “Now I’m a little concerned whoever the president might be, but that gives you total control.”

The Committee on Foreign Investment in the United States, a panel of government agencies that reviews international deals for international security concerns, sometimes requires companies to agree to terms that ensure that a transaction does not compromise American supply chains.

In the case of U.S. Steel and Nippon Steel, the scrutiny faced criticism for being politicized. After the committee, known as CFIUS, failed to reach a consensus on the deal before Mr. Biden blocked it, Mr. Trump directed it to conduct a fresh review in April.

“This isn’t normal CFIUS procedure,” Aaron Bartnick, a former CFIUS official in the Biden White House and Treasury Department, said of the long — and at times conflicting — review for the U.S. Steel deal.

The decision by Mr. Trump aligns with his “America First” investment policy and his general desire to cut deals. The president has made attracting foreign investment to the United States a top priority, and he has deployed tariffs and tax cuts to lure international firms to build things in America.

When Mr. Trump announced last month that he intended to approve the deal, he also announced plans to raise tariffs on steel and aluminum imports to 50 percent from 25 percent, putting additional pressure on Nippon Steel to complete its terms so it would have access to the American market through U.S. Steel.

While details about how the golden share will work remain scarce, national security experts said giving the U.S. a stake in transaction of this sort would mark a significant departure in how the country treats transactions.

“It would be very odd for the United States to start holding golden shares in companies,” Gregory Shaffer, a law professor at Georgetown University, said. “This is something the U.S. has long complained about” with other countries, he added.

Aimen Mir, a deputy assistant secretary for investment security at the Treasury Department during the Obama administration and the first Trump administration, said a golden share granting explicit government control of the company could set a new precedent for foreign investment.

“The notion of the U.S. government taking an ownership interest or extracting an ownership interest to get approval would be concerning,” Mr. Mir, who leads the CFIUS practice at the Freshfields law firm, said. “That opens up a new door.”