The following month, Trump gave Cotton a victory on the touchstone issue of his Senate career by decertifying Iran’s compliance with the nuclear-arms deal that the Obama Administration had negotiated. “I told the President in July that he shouldn’t certify that Iran was complying with the agreement,” Cotton told me. “Putting aside the issue of technical compliance or noncompliance, it’s clear that the agreement is not in our national interest.” Following Trump’s action, Cotton joined forces with Senator Corker, the chairman of the Foreign Relations Committee, on a proposal that, if passed, would likely lead to the termination of the Iran nuclear deal and the reimposition of American sanctions.

“Let there be no doubt about this point,” Cotton said, in a recent speech to the Council on Foreign Relations. “If we are forced to take action, the United States has the ability to totally destroy Iran’s nuclear infrastructure. And, if they choose to rebuild it, we could destroy it again, until they get the picture. Nor should we hesitate if compelled to take military action.” In describing his preferred approach to negotiations with Iran, Cotton said, “One thing I learned in the Army is that when your opponent is on his knees you drive him to the ground and choke him out.” In response, a questioner pointed out that killing a prisoner of war is not “American practice.” (It is, in fact, a war crime.)

Similarly, in North Korea, Cotton supports Trump’s brinkmanship with Kim Jong Un, and excoriates China for its failure to rein in its ally. “Time and time again, Beijing shows that it is not up to being the great power it aspires to be,” Cotton said. (His hostility toward China endears him to the Bannon wing of the Republican Party, which views the U.S.-China relationship as the defining conflict of the modern world.)

Cotton has emerged as such a close ally of the Trump White House that one recent report suggested that the President would name him director of the C.I.A. if Mike Pompeo, the current director, were to replace Rex Tillerson as Secretary of State. (Trump is widely believed to be dissatisfied with Tillerson.) In a conversation in mid-October, Cotton did not dismiss the possibility of taking the C.I.A. job. “I am pleased to be a senator,” he told me. “But, of course, I will always take a call from the President, and he has called me many times.” As a member of Trump’s Administration, Cotton would ratify the President’s instincts. He offers Trump a certainty that matches his own, especially about the threats the nation faces and the best ways to address them.

In August, I visited Cotton in the house where he grew up, in Yell County, Arkansas. When I arrived, Cotton’s father was also walking in the door. Len Cotton did not offer to shake hands right away, because he had just welcomed two newborn calves to the family farm, and he thought it prudent to wash up first.

The Cottons have been in Arkansas for six generations, and Tom’s parents make their living running what’s known as a cow-calf operation, on several plots of land in the Arkansas River Valley. In the specialized world of beef-and-dairy production, the Cottons’ business is the first stage—the production of the cows, which are sold to ranchers. The Cottons have always done some farming, but when Tom was a boy his mother was a public-school teacher and a middle-school principal, and his father worked for state government, doing inspections for the Department of Health. Like most people in Arkansas at the time, Tom’s parents were Democrats. But the leitmotif of Tom Cotton’s political career has been the decline of the Democratic Party among white voters in Arkansas. “The Democratic Party has drifted away from them,” Tom told me, as his parents sat nearby. “Bill Clinton would be repudiated by his own party today. Hillary Clinton repudiated a lot of her husband’s chief accomplishments when he was in office. So that’s a real fundamental story about politics in Arkansas and politics across the heartland.”

Tom had an idyllic boyhood in the town of Dardanelle, centered on sports and school, where he excelled, and he won admission to Harvard. When he arrived in Cambridge, in the fall of 1995, he still had braces on his teeth, though he had grown to a full six feet five; friends remember him as a bit of a loner, at least at first. He was also already a conservative, if not a Republican, as he was not afraid to let his new neighbors know.

Cotton began writing an opinion column for the Crimson, the campus daily, where he made a name for himself as an outspoken dissenter on a liberal campus. Shortly before he graduated, in 1998, Thomas B. Cotton wrote a farewell to his readers. “I never sought to be loved or to be treated justly,” he said. “How could I? I wrote against sacred cows, such as the cult of diversity, affirmative action, conspicuous compassion and radical participatory democracy. I wrote in favor of taboo notions, such as Promise Keepers, student apathy, honor and (most unforgivably) conservativism.” After college, Cotton went to Harvard Law School. He worked in law firms during the summers and landed a clerkship with a federal appeals-court judge.

He appeared headed for a life of prosperous anonymity in law, but the attacks of September 11, 2001, upended his plans. “I was going to play intramural basketball, enjoy my last year in school, and then, in the second week of school, the attacks happened, and that changed my orientation,” he told me. “I spent a lot more time from that point forward thinking about the threat we faced, reading about history, reading military history, started thinking about joining the Army.”

Cotton approached the matter with the careful deliberation that has characterized his career. He decided to take the clerkship he had already accepted, with Judge Jerry Smith, on the Fifth Circuit, then work at Gibson, Dunn & Crutcher, in Washington, to start paying off his student loans. “I thought he had a great future at the firm,” Bill Kilberg, the partner who supervised Cotton’s work, told me. “Then, one day in 2004, Tom came and said he was thinking about leaving the firm to join the Army. I said, ‘Tom, the Army has plenty of lawyers, they really don’t need you, and it’s not necessary for you to join the Army to serve.’ He said, ‘Oh, no, I’m not going to be a lawyer. I’m going to be an Airborne Ranger.’ And I looked at him and said, ‘Tom, have you talked to your mother about this?’ ”

When I asked Cotton about the decision, he said, “The Army needs lawyers, but that’s not the heart of the Army’s mission. The Army’s mission is the infantry’s mission.” He went on, “So I wanted to do that mission, wanted to do the heart of it. I wanted to lead troops in combat.”

Cotton served with the 101st Airborne Division in Iraq. He led ninety-six-hour patrols in the field, followed by thirty-six-hour stretches at a base near Baghdad. The base had Internet access, and one day in the summer of 2006 Cotton saw that the Times had disclosed, over objections from the Bush Administration, the existence of certain terrorist-surveillance programs. Cotton fired off a letter to the editor, copying several conservative Web sites, and then left on a patrol, where he was cut off from all electronic contact with the United States. “Congratulations on disclosing our government’s highly classified anti-terrorist-financing program,” the letter begins. “I apologize for not writing sooner. But I am a lieutenant in the United States Army and I spent the last four days patrolling one of the more dangerous areas in Iraq.”

The letter combined outrage, overstatement, and savvy politics in a manner that Trump would perfect a decade later. “You may think you have done a public service, but you have gravely endangered the lives of my soldiers and all other soldiers and innocent Iraqis here,” Cotton wrote. “Next time I hear that familiar explosion—or next time I feel it—I will wonder whether we could have stopped that bomb had you not instructed terrorists how to evade our financial surveillance.” He continued, “And, by the way, having graduated from Harvard Law and practiced with a federal appellate judge and two Washington law firms before becoming an infantry officer, I am well-versed in the espionage laws relevant to this story and others—laws you have plainly violated. I hope that my colleagues at the Department of Justice match the courage of my soldiers here and prosecute you and your newspaper to the fullest extent of the law. By the time we return home, maybe you will be in your rightful place: not at the Pulitzer announcements, but behind bars.” When Cotton returned to the base, he learned that the Times hadn’t run the letter but the Web sites had, and the chief of staff of the Army had distributed it to his subordinates.

“I started hearing about Tom when he was still in the military, when I was state chair of our party,” Dennis Milligan, who is now the Arkansas state treasurer, told me. “As chair, you’re always looking for new talent, and people were talking about him even then. They knew he had given up all that money in the law to serve his country.” From Iraq, Cotton was summoned to serve in the Old Guard. (Cotton hoped he had won appointment to the prestigious unit on merit, but the Army had simply summoned the six tallest lieutenants in Iraq.) Later, he volunteered for a tour in Afghanistan, where he won a Bronze Star, before leaving the service, in 2009. After a brief stint working at McKinsey, Cotton returned to Arkansas to run for Congress in his home district. Mike Ross, a Democrat, had retired, and Cotton, campaigning with a heavy emphasis on his military service, won the open seat with about sixty per cent of the vote.