Jonathan Bernstein offers a plausible explanation for why hawks are dominating the intra-Republican debate at the moment:

No current war (that is, a war with significant American casualties) means no current antiwar movement, so everyone in the Republican Party who cares about foreign policy will be an interventionist of some kind — and therefore strongly anti-Rand Paul. Peace is bad for the peaceniks.

That is certainly a big part of it, but Bernstein may still be overstating things quite a bit. He is obviously right that an antiwar message is more appealing and salient when the U.S. is fighting a prolonged ground war. The longer such a war goes on without success, the more appealing that message will usually be. Not only will there be more support for candidates that criticize the management of the war, but there will be increased support for advocates of withdrawal and candidates that were opposed to the war from the start. Support for both the Iraq and Afghanistan wars declined and then collapsed as casualties increased. Once those wars have been concluded, or at least once U.S. involvement in them ends, they naturally matter less in our political debates here at home. There is normally no political mobilization against relatively low-cost air wars such as Libya or the current war against ISIS. As long as the U.S. limits its involvement in foreign conflicts so that there are few or no American casualties, it is undoubtedly harder to mobilize opposition to them, and for that reason it is also harder to sell voters on foreign policy restraint that counsels not fighting those wars in the first place. I agree with Bernstein up to this point.

However, the fact that the U.S. is very secure ought to work to the advantage of candidates that favor restraint and peace. Because the U.S. faces only manageable and minor threats, there is no compelling reason for the U.S. to have the activist, costly, and heavily militarized foreign policy that we have. That is why hawks are continually exaggerating the scale of foreign threats and the degree to which the world is becoming less stable. They need to make voters believe that constant U.S. activism is necessary, but it is very often just the opposite. The Iraq and Afghanistan wars have also had some lasting effects on the foreign policy debate both inside and outside the GOP. There are many more skeptics of military action on the right than there were ten or fifteen years ago, and there is more room on the right for opposition and criticism of foreign wars than there was a decade ago. That certainly hasn’t eliminated the significant advantages that hawkish candidates still enjoy inside the GOP, but it has made reflexive hawkishness more of a potential liability for Republican candidates with their own primary voters (and with voters in a general election) than it has been in a long time.

As it is, there is still a significant constituency inside the GOP that thinks the U.S. is too activist overseas and believes it shouldn’t be doing as much as it does:

37% of Republicans said just a few months ago that the U.S. does too much to “solve world problems,” and it wasn’t that long ago that more than half said this. That suggests that at least a third of Republicans remain open to arguments in favor of restraint. That doesn’t necessarily mean that this is a high priority for all of these Republicans (it probably isn’t), nor does it guarantee that they would all support a candidate promoting realism and restraint, but it suggests that there is a potential base of support there for a candidate that makes the attempt.

If the rest of the Republican field is made up of many hawks, they will be splitting up the hawkish voters among themselves, and that creates at least a small opening for an advocate of restraint to slip through. After all, a pro-restraint candidate doesn’t have to win most of the voters that care most about foreign policy to be competitive. There just have to be enough primary voters that are open to supporting such a candidate and don’t want to reject him because of his foreign policy views. Whether a pro-restraint candidate can win those voters over depends on how much of the rest of his agenda they support and how much they trust the candidate’s judgment.