WASHINGTON » First there was Sen. Rand Paul of Kentucky, who held up bills, conducted an anti-drone talk-a-thon and generally annoyed his colleagues. Next was Sen. Ted Cruz of Texas, who whipped up a campaign that shut down the government.

Now comes Sen. Tom Cotton of Arkansas, another Republican freshman, whose letter to the leaders of Iran warning against a nuclear deal with the Obama administration has caused an international uproar.

At 37, Cotton is the youngest member of the Senate and had served as of Wednesday exactly 65 days. A graduate of Harvard and Harvard Law School, Cotton served as an infantry officer in the 101st Airborne Division in Iraq in 2006, one of the bloodiest periods of the war.

So while Paul and Cruz influenced the Tea Party, libertarian wing of their party, Cotton personifies a wave of Republican newcomers to the Senate who back a hawkish, interventionist foreign policy.

"Tom Cotton is ahead of the mainstream of Republicans on foreign policy thinking," said Bill Kristol, the editor of the conservative Weekly Standard and an early supporter of Cotton’s political career. "Most of those running in 2016 will sound a lot more like Cotton than Rand Paul."

If Cotton and other newcomers wish to lead on military and foreign affairs issues, then "more power to them," said Sen. John McCain of Arizona, who once referred to Paul and Cruz as "wacko birds." "I was a bit of an upstart myself."

The word most used to describe Cotton — by his admirers and detractors — is disciplined. A long-distance runner, he begins most days with a long workout and focuses relentlessly on his daily tasks. His seriousness is such that he was often dinged during his campaign as seeming more like a robotic agriculture display than a Senate hopeful.

Willowy and alpine, with a pronounced Adam’s apple and a mild Southern drawl, Cotton, who declined to be interviewed for this article, regularly sails past reporters in the Senate hallways in what appears to be a hypnotic trance. He is unflappable, even during a grilling by Megyn Kelly of Fox News about his letter, which was signed by 47 Republican senators and quickly dismissed in Tehran.

"What’s the point in writing to the Iranian mullahs?" Kelly demanded of him Tuesday. "They already dismissed it, like, ‘Whatever.’"

The Obama administration has similarly hammered Cotton. "My reaction to the letter was utter disbelief," Secretary of State John Kerry said at a Senate hearing on Wednesday, warning that the letter could embolden Iranian hard-liners.

Kerry did acknowledge one of the points of Cotton’s letter, that any deal would not be "legally binding" and that the next president could revoke it. But, Kerry said, no president would do that as long as Iran kept its part of the bargain and as long as the other negotiating partners — Britain, France, Germany, Russia and China — continued to support it.

"I’d like to see the next president, if all of those countries have said this is good and it’s working, turn around and just nullify it on behalf of the United States," Kerry said sarcastically.

Cotton has a long history in conservative thought.

Raised on his parents’ cattle farm in Dardanelle, Ark., he stood out for — along with his basketball skills — his studiousness and curiosity about Republican politics, particularly in his Democratic family. He participated in a program that brought young students to Little Rock, the state capital, said his childhood friend, Michael Lamoureux, who is now the chief of staff to the Arkansas governor, Asa Hutchinson.

"I don’t know where he got his ideas," Lamoureux said. "Probably from reading."

At Harvard College, Cotton wrote a 92-page senior thesis on the Federalist Papers and later served as a clerk with the U.S. Court of Appeals before joining a private law practice in Washington.

After the terrorist attacks of Sept. 11, 2001, Cotton joined the Army and spent almost five years on active duty, including as a platoon leader on combat patrols in Baghdad. While in Iraq, Cotton reached out to Kristol to compliment him on his magazine, Kristol said, and began the cultivation of other conservatives.

That same year, reporters at The New York Times broke a story about the Bush administration’s tracking of terrorist financing, and Cotton wrote a letter to the paper expressing his anger: "By the time we return home," he wrote, "maybe you will be in your rightful place: not at the Pulitzer announcements, but behind bars."

The Times did not print the letter but it went viral on conservative websites and helped launch Cotton’s national profile.

Back home, Cotton began to attend events at the conservative American Enterprise Institute and became closer to Kristol and others who saw his potential. After his military service, he took a job in 2009 as a management consultant for McKinsey & Co. and began pondering a run at a Senate seat back home against Sen. Blanche Lincoln, a Democrat.

"It would have been a reach," Kristol said, so Cotton deferred until a House seat opened in 2012. "He stayed genuinely close to people in Arkansas," Kristol added. "A lot of this was generated from bottom up more than at the Mayflower Hotel bar in D.C."

Leaders in the House noted Cotton’s intellect and the fact that he voted without fail against tough bills. In 2014, he campaigned in a camouflage-themed recreational vehicle against the Democratic incumbent, Sen. Mark Pryor, and handily beat him.

While Cotton was broadly supported by Senate Republicans, his letter attracted many critics in his own party and beyond. "The idea of engaging directly with foreign entities on foreign policy is frankly a gross breach of discipline," said Maj. Gen. Paul D. Eaton, who is retired and a senior adviser to VoteVets, a liberal political action committee consisting of veterans.

Sen. Bob Corker of Tennessee, the chairman of the Senate Foreign Relations Committee, declined to sign the letter. "Tom is a really nice person, and I know feels very strongly, especially about the issue of Iran," he said. "And that’s all I can say."