Arkansas elected Republican Rep. Tom Cotton to the Senate on Tuesday, sending incumbent Mark Pryor packing. You might not be surprised that a Republican won in Arkansas in a heavily GOP year, but Cotton's victory is a much bigger deal than you probably think. His election is a significant step for the future of GOP foreign policy, and thus perhaps for American foreign policy.

(Read more: Sen. Rand Paul just gave one of the most important foreign policy speeches in decades)

Cotton, you see, is the golden child of the Republican party's hawkish establishment. He still calls the 2003 Iraq invasion a "just and noble" war. He's young — just 37 — and fervently backed by some of the most influential conservative figures in the nation. His Senate victory makes him a serious candidate for an even higher office some day. But even before then, his ascent could represent a larger movement in his party's foreign policy.

This sets him on a collision course with the GOP's other leading young voice on foreign policy: Sen. Rand Paul. One of Paul's top priorities is moving the Republican Party away from George W. Bush's neoconservatism; one of Cotton's is pulling the party back towards it. And given the slate of immediate foreign policy issues facing the Senate, the two are likely to be at odds sooner rather than later.

Cotton's hawkishness is his defining political trait

Like much of the GOP class of 2014, Cotton is extremely conservative on domestic policy. He scored a 92 on the influential Club for Growth's House scorecard last year, a rough approximation of a member's conservatism measured by their votes on economic legislation. That 92 puts him in the top 5 percent of most conservative House members.

But it's foreign policy where Cotton really distinguishes himself from the pack. As the Atlantic's Molly Ball breaks down in a fascinating profile, the Senator-elect's career began in 2006 with his criticism of the New York Times for revealing a clandestine US spying program targeting terrorist finances. "By the time we return home," he wrote to the Times reporters, "maybe you will be in your rightful place: not at the Pulitzer announcements, but behind bars."

"I think that George Bush largely did have it right"

At the time, Cotton was a lieutenant serving in Baghdad. When his letter was published on the conservative blog Powerline, it went the 2006 equivalent of viral and Cotton became a conservative media darling. He began corresponding with Bill Kristol, the editor of the flagship neoconservative publication, the Weekly Standard. According to Ball, Kristol and Cotton developed what the former calls "a bond beyond pure policy" over their shared foreign policy views. Like Kristol, Cotton believes that aggressive American force against rogue states and terrorists is the only way to keep the world safe.

"I think that George Bush largely did have it right," Cotton said, "[in] that we can't wait for dangers to gather on the horizon, that we can't let the world's most dangerous people get the world's most dangerous weapons, and that we have to be willing to defend our interests and the safety of our citizens abroad even if we don't get the approval of the United Nations."

Cotton's foreign policy hawkishness, and his backing from the neoconservative establishment it's brought, have helped shape his political persona. "Cotton has staked his young political career on a staunchly assertive, activist view of American military power," Politico's Alexander Burns wrote in a 2013 profile. For conservatives who support an aggressive foreign policy, "there is no Republican under the age of 40 with more riding on his career than Cotton," Burns concludes.

Rand Paul and Tom Cotton are natural rivals

If Senator Cotton will be the avatar for a new generation of neoconservatives, then Rand Paul seems bound to quickly become his greatest foe within the party.

Paul's late October speech on world politics, widely understood to be a preview for his 2016 foreign policy platform, took a number of undisguised shots at people with Cotton's views. "We will not instigate war," Paul said. "Americans yearn for leadership and for strength, but they don't yearn for war."



While Paul wants to put tight legal limits on America's targeted killing program, for example, Cotton supports expanding the drone war. While Paul opposes getting more involved in the wars in Syria and Iraq, Cotton is open to deploying ground troops to fight ISIS. On essentially every major issue of war and peace facing the country, Cotton and Paul's instincts run opposite ways.

The contrast even extends to their styles and backgrounds. Paul acts like an everyman; Cotton boasts two Harvard degrees. Paul jokes about being an ophthalmologist dabbling in foreign policy; Cotton talks up his military career.

These differences are more than just stylistic. Paul's views on foreign policy are idiosyncratic; he's the heir to a long and diffuse tradition of grassroots non-interventionism on the American right, but is trying to invent a new version of the doctrine that can appeal to the American mainstream.

Cotton, meanwhile, studied the neoconservative icon Leo Strauss at Harvard. Cotton's Strauss-inflected senior thesis amounts to an extended defense of the theory that intellectually gifted elites should rule. "Such men rise from the people through a process of self-selection since politics is a dirty business that discourages all but the most ambitious," he writes. Now, that sentence was intended merely as a description of the views of some American founders. But according to Ball, the entire thesis is dedicated to defending precisely their ideas about the superiority of public officeholders.

Paul, on the other hand, is a populist, including in foreign policy, attempting to harness public discontent with America's last decade of war for his campaign to remake the Republican party. Cotton, meanwhile, represents the Republican elite's best face: a smart, polished, consummately connected advocate for America's national security state. And now the two will be facing off on the Senate floor.

They're about to come to blows over Iran and ISIS

It shouldn't take very long for the latent Paul-Cotton tension to become overt political conflict. Immediately after the midterm elections, the Senate is going to be forced to address two major foreign policy issues. Both will likely still be unresolved by the time that Cotton takes office in January.

November 24 is the latest deadline for reaching a deal in the ongoing negotiations between the United States and Iran over Iran's nuclear program. The two countries will likely miss the deadline, but either way Iran's nuclear program will likely be a defining foreign policy issue in Obama's second term.

Congress has a huge role to play on Iran. The American negotiating position is that, if Iran agrees to sufficiently limit its nuclear development, the United States will lift economic sanctions and move towards more normal diplomatic relations. But Iran sanctions are enshrined in legislation. If Congress wants, it can prevent Obama from relaxing sanctions. It can even impose new sanctions while Obama is negotiating, potentially torpedoing the talks by convincing Iranians that the president can't deliver on sanctions relief.

There's no doubt that the two will come to blows in a high-profile way

Naturally, Cotton and Paul have opposing views here. Paul supports a negotiated solution to the Iran crisis. Cotton said that the interim agreement with Iran reached last fall was worse than Munich.

The question of what to do about ISIS will also put Paul and Cotton at odds. House Speaker Boehner wants to save a vote on giving legal authorization for Obama's military campaign against the group until after the lame duck session — that is, after Cotton takes up his position in the Senate. That means that, even if the Senate votes on the issue sooner, the Republican-controlled Senate will likely have to revisit authorization in January.

In this debate, Paul will likely push for the most limited possible language, while Cotton will be pressing to give the executive as much leeway to wage war as possible. Both care about this issue, care about getting some foreign policy profile, and think about US policy toward ISIS very differently.

And there will be other issues. Funding for Syrian rebels, the Ukraine crisis, and NSA spying could all conceivably come to the front of the national security docket. Each one will likely see Paul and Cotton on opposite sides, attempting to sway their fellow Senators and the GOP base to their point of view.

The stakes: the GOP's foreign policy soul

For the next two years, Paul will be laying the groundwork for his 2016 run for president. In order to do that, he'll need to persuade Republican elites — donors, media figures, political operatives, and the like — that he can and should win. This means selling them on his foreign policy vision.

Cotton may well prove one of the biggest obstacles to Paul's campaign. At the same time that Paul is attempting to push the party in a non-interventionist direction, or at least sell the party on letting him possibly lead them that way, Cotton will be tugging as hard as he can in the other direction. Either overtly or implicitly, his efforts will work against Paul's shadow campaign. The more interventionist the GOP center of gravity is, the more danger Paul faces in the so-called "shadow primary" where elites decide whom they ought to support.

So this is about Paul's bid for the presidency — and, by extension, the GOP's foreign policy soul. If Paul wins the primary, then the GOP and its elected officials will have to line up behind him. That will mean defending his foreign policy against Democrats, who will likely blast Paul from an interventionist point of view, especially if Hillary Clinton is the Democratic candidate. The entire Republican center of gravity on foreign policy could shift.

Republicans such as Cotton will be much less welcome in that GOP. So they'll fight Paul tooth and nail to make sure it won't happen. Cotton's ascension to the Senate, then, opens one more very prominent front in the Republican civil war.