Mr. Gertler arrived in Congo in 1997 as a 23-year-old diamond dealer, determined to challenge the global giant in supplying raw diamonds, the South African-based De Beers.

One of his first big breaks came about three years later, when Laurent Kabila, then the president of Congo, needed weapons to wage a war that would last for more than a decade.

Offering monopolies to foreigners looking to tap into Congo’s rich mineral resources was a way for Mr. Kabila to raise cash needed to fight the war. Among them was a deal to export diamonds with Mr. Gertler, who was considered an appealing intermediary because of his ties to generals in the Israeli Army that could help Congo procure weapons, according to two reports issued by the United Nations in 2001. (Mr. Gertler disputed the findings.)

But the U.N. concluded that Mr. Kabila used money gained selling access to the nation’s mineral wealth — including his deal with Mr. Gertler — to expand the Congolese military forces, a move that helped popularize the terms “conflict diamonds” and “blood diamonds.”

“Conflict diamonds are exchanged for money, weapons and military training,” a U.N. report describing Mr. Gertler’s work said.

Mr. Gertler was also indirectly accused, in a Justice Department court filing in 2016, of paying more than $100 million in bribes to government officials in Congo on behalf of a company named Och-Ziff “to obtain special access to and preferential prices for opportunities in the government-controlled mining sector.”

A spokesman for Mr. Gertler, Aron Shaviv, said Mr. Gertler was never interviewed or charged in the case and he denied any wrongdoing. Instead, Mr. Shaviv said, Mr. Gertler’s companies have directly invested more than $1.5 billion in Congo, becoming one of the nation’s largest employers and taxpayers, starting when no other foreigners were willing to take the risk of doing business in the middle of a war.

“He did buy cheap and he may sell at a much, much higher price because he made the investment when no one else did, no one else would dare go to Congo,” said Mr. Shaviv, a political consultant who served as Mr. Netanyahu’s campaign manager in 2015.

“He played a very pivotal role in not only advising Kabila, but also sort of speaking with authority and definitely carrying the United States’ message,” Jendayi E. Frazer, who then served as an adviser to Mr. Bush for African affairs, said in an interview.

Mr. Gertler’s work helped lead to a peace deal in 2003. And it also cemented his relationship with Joseph Kabila. The Congolese government began to grant new deals to Mr. Gertler and his growing empire of companies, which expanded from diamonds into copper, cobalt, oil, gas and gold.

In just five deals negotiated between 2010 and 2012 to sell copper and cobalt through offshore companies linked to the Fleurette Group, which is controlled by Mr. Gertler and his family, the citizens of Congo lost an estimated $1.36 billion because the nation’s resources were being sold at one-sixth of their value, according to a report prepared in 2013 by Kofi Annan, the former U.N. secretary general, and other prominent African officials.

The forgone revenues to Congo from the deals “were equivalent to more than double the combined annual budget for health and education,” the report concluded.

In Congo, over 70 percent of the population lives in extreme poverty, with an income of less than $1.90 a day. But the profits generated for Mr. Gertler were extraordinary, averaging 512 percent, according to the study, turning him into one of the 29 youngest billionaires in the world, according to Forbes.

It was not just Mr. Gertler who was reported to be becoming tremendously wealthy through these deals.

Joseph Kabila, who had called Mr. Gertler “an old and trusted friend,” was also taking a cut, according to a 2017 report by the nonprofit Congo Research Group, which found that Mr. Kabila and his family ended up as owners in part or whole of at least 80 companies. Among them was one mining deal with a business linked to Mr. Gertler that drew so many questions from the International Monetary Fund that the fund canceled disbursement of more than $200 million in loans it had planned to make under an agreement with Congo.

The corruption and exploitation inherent in these types of deals were just the sort that a new appointee at the Treasury Department named Sigal P. Mandelker was determined to confront when she was confirmed as the top official in charge of sanctions enforcement in 2017.

“Our objective is to change behavior, inspire democracy and freedom, and disrupt the ability of kleptocrats, human rights abusers and others from stealing the wealth of their country,” Ms. Mandelker said in a 2019 speech.

Ms. Mandelker drew bipartisan praise for her effort to take advantage of new authority Congress granted to the Treasury in 2016. The Global Magnitsky Human Rights Accountability Act, as the law is known, is named after a Russian tax lawyer, Sergei Magnitsky, who died in a Moscow prison in 2009 after he exposed corruption by Russian officials.

The new law allowed the Treasury to freeze the assets of individuals or businesses operating anywhere in the world that were engaged in “gross violations of internationally recognized human rights.”

Working with the State and Justice Departments, Ms. Mandelker’s team included Mr. Gertler in the first round of individuals penalized in December 2017, citing his record of “opaque and corrupt” mining and oil deals in Congo. A second round of sanctions in 2018 targeted more companies affiliated with Mr. Gertler.

The sanctions on Mr. Gertler severely constrained his ability to do business around the world by cutting off his access to the United States banking system and limiting his access even to non-U.S. financial institutions concerned about running afoul of the American law.

But less than a year after the sanctions were imposed, Mr. Gertler began his campaign to roll them back.

The push started with a seemingly innocuous request: Grant Mr. Gertler permission to use some of his money to make charitable donations to hospitals, libraries and schools in Congo.

But even that plan drew concern from some State Department officials, who were worried that the donations would allow Mr. Gertler to bolster his standing in Congo and help supporters of Mr. Kabila, by then out of office, challenge efforts by the new, democratically elected president, Félix Tshisekedi, to assert control.

By last year, Mr. Gertler was also battling to rebut a report by two human rights groups citing what they said was evidence that he was evading the sanctions by using a network of shell companies, frontmen and proxy bank accounts to move millions of dollars in and out of Congo and even to acquire new mining rights there.

Mr. Gertler sued both the human rights groups and the Israeli newspaper Haaretz, which published reports detailing the allegations. Lawyers working for Mr. Gertler and a bank in Congo claimed the reports were based on documents that were stolen and then tampered with. The paper and the human rights groups have defended the accuracy of their reporting.

Instead of supporting Mr. Gertler’s bid for permission to make charitable donations, State Department officials responsible for Africa pressed the Treasury Department to expand the sanctions.

But by the end of 2019, key players at the Treasury, including Ms. Mandelker, had started to leave the Trump administration, and State Department officials like Mr. Pham said they found it more difficult to get new Magnitsky sanctions imposed.

The officials turned to the Senate Foreign Relations Committee for help in keeping up the pressure on Mr. Gertler. In August, members of the committee sent the Treasury Department a bipartisan letter that did not mention him by name but carried a clear message.

To help build democracy and fight corruption in Congo, the letter said, the United States “should designate additional officials and companies responsible for or complicit in high-level corruption, including the misappropriation of state assets, for targeted financial and travel sanctions.”

But Mr. Gertler’s team, including Mr. Dershowitz and Mr. Freeh, had a different message. They had solicited a letter from Ms. Frazer attesting to Mr. Gertler’s role in the peace negotiations nearly two decades earlier and distributed it to Trump administration officials. As far back as 2019, they set up meetings with State Department officials, making the case that his activities had helped the interests of the United States.

“His first effort was a lobbying effort,” Mr. Shaviv said of Mr. Gertler’s campaign.

But Treasury rules state that “professional services such as lobbying, public relations, government affairs, consulting and business development are not legal services, and are generally not covered” by an exemption that allows people under sanctions to hire lawyers.

Mr. Dershowitz said the meetings were permitted because he did not lobby the White House or others on this matter.

“My role was purely limited to the legal issues,” Mr. Dershowitz said.

But with time running out on Mr. Trump’s tenure and the sanctions still not lifted, Mr. Gertler decided to make a strategy shift. While not admitting any past wrongdoing, Mr. Gertler’s lawyers told the Treasury Department in early December that he was prepared to take any reasonable steps to assure the United States that he would abide by the law, including hiring outside monitors and submitting detailed periodic reports on financial transactions.

“Our entire approach was to assure them that going forward, there would be no problem,” Mr. Shaviv said.

At the same time, assertions were being made that Mr. Gertler had been of value to U.S. intelligence agencies.

“It’s absolutely the case that the national security interests of both Israel and the United States were implicated in this,” Mr. Dershowitz said, although he and others declined to provide any specifics. Mr. Shaviv declined to discuss whether Mr. Gertler had undertaken any such activities, but said that if they did take place, they would be described as “services rendered to the United States of America.”

Whatever Mr. Gertler did that benefited the United States was sensitive enough that Israeli officials said they were aware of it but declined to comment on its nature. Two Israeli officials told The Times that the United States had informed Israel that in line with a decision by Mr. Mnuchin and Mr. Pompeo, the terms of the sanctions imposed on Mr. Gertler would be eased “out of reasons of American national security.”

But several former State and Treasury Department officials said that while as a foreigner operating in Congo Mr. Gertler might have had information the United States considered valuable, keeping him on the sanctions list also had a value to Washington by helping to promote the anti-corruption effort.

“The only value to national security that Gertler has comes from him being placed in the box that he was put into with the sanctions,” Mr. Pham said.

In any case, the decision to grant him the one-year license was unusual in a number of respects, they said.

The Treasury Department traditionally agrees to revoke sanctions only after individuals have proved they have already changed their behavior, not simply agreed to make such changes in the future, said Mr. Smith, the former head of the sanctions unit, who is now a national security lawyer at the law firm Morrison and Foerster. Mr. Gertler had not previously provided the United States such evidence.

Furthermore, if Mr. Gertler’s assets in U.S. banks were going to be unfrozen and his corporate entitles allowed to once again do business with United States financial institutions, as the license allowed, that kind of deal would almost certainly need to be made public, not issued in secret as this one was. This kind of review also typically takes months of effort, not the six weeks that it took in this case.

“This is a unique, one-of-a-kind response that you don’t see with the United States government,” Mr. Smith said of the so-called specific license that Mr. Gertler received. “It is the most shocking license I have ever seen in a few decades of working on economic sanctions.”

When word of the decision to grant Mr. Gertler the one-year license eventually trickled out after Mr. Trump left office, it set off a firestorm of criticism from officials who said it would undercut efforts by the United States to fight corruption.

Mr. Hammer, the U.S. ambassador to Congo, was at first so confused at the news, according to one State Department official briefed on the matter, that he called officials in Washington to figure out if a mistake had been made.

“This has made my job much tougher,” an angry Mr. Hammer told colleagues.

House and Senate Democrats fired off letters to the Treasury and State Departments. A coalition of 30 Congolese and international human rights groups assailed the move, with one of the letters calling the move a “terrible blow to the heart of one of the most lauded and effective anti-corruption programs of the last decade.”