Sen. Bernie Sanders, the Vermont socialist challenging Hillary Clinton for the Democratic presidential nomination, is the activist left's favorite presidential candidate. Sanders's willingness to champion ideas that most of Washington won't touch — single-payer health care, for example, or massive across-the-board spending hikes — has won him an adoring fan base, especially online.

Sanders is the only self-described socialist in the Senate and has defined himself by challenging the Democratic establishment from the left. But there's one area where Sanders hasn't taken quite as independent of a course: foreign policy.

On foreign policy, he's toward the leftward end of the Democratic party, but still well within Washington's conventional norms. That's drawn him some condemnation on the left, particularly with regards to his views on Israel.

Bernie Sanders's positions on war have been controversial on the left

Among actual socialists, Sanders's mainstream foreign policy views are no secret. "Sanders doesn’t offer the ... principled anti-imperialist politics that we should demand on the Left," Jacobin editor Bhaskar Sunkara writes in an otherwise friendly editorial. A more critical response from Ashley Smith is more blunt: "his foreign policy positions are to the right of many liberal Democrats."

That's perhaps overstating the case: Sanders is neither substantially more conservative, nor more liberal, than a standard liberal Democrat.

Take the war on ISIS, for example. Like many Democrats, Sanders has vocally opposed deploying US combat troops to Iraq or arming Syrian rebels. But he's been pretty comfortable with the US bombing campaign against ISIS: "I have supported U.S. airstrikes against ISIS and believe they are authorized under current law," the senator said in a February statement, and wants regional Arab powers to take the lead in waging an international ground war against ISIS.

Sanders's tension with the left on foreign policy goes back some time. As a congressman, he voted for a resolution supporting the 1999 US air campaign in Yugoslavia — a vote that prompted both a pleading Nation editorial asking him to reconsider and a nasty public resignation letter from Sanders staffer Jeremy Brecher. "Is there a moral limit to the military violence you are willing to participate in or support?" Brecher asked his former employer. "Where does that limit lie?"

Sanders's willingness to endorse wars has varied over his career, but he is not a pacifist. He voted for the 2001 Authorization for the Use of Force in Afghanistan, but opposed Obama's 2009 troop surge in the country. He opposed both the Gulf War and the 2003 invasion of Iraq. He had "reservations" about the 2011 war in Libya, but supports drone strikes on suspected terrorists "in a very selective way."

On this issue, then, Sanders isn't far off from your average liberal Democrat. He's generally skeptical of the use of force, but willing to endorse it in very narrow and limited cases where he thinks it could save lives or advance American interests. That doesn't make him a warmonger who is "to the right of many liberal Democrats," but it is a notable divergence in his reputation as a champion of the left and challenger of the Washington status quo.

Sanders is pro-Israel, but he doesn't emphasize it

The use of force isn't the area in which Sanders and the activist left have the most conflict. That'd be Israel.

"When I see Senator Bernie Sanders, I see someone who is a typical pro-Israel Jewish Democrat," Aaron Keyak, a Democratic political consultant familiar with Israel issues, told the Forward's Josh Nathan-Kazis. "He prefers for Israel to have a left of center government, but he still fundamentally supports Israel."

Sanders supports a two-state solution, one for Israelis and another for Palestinians. While he can be critical of Israel, he does not refrain from criticizing Palestinians as well.

"The Palestinians must fulfill their responsibilities to end terrorism against Israel and recognize Israel’s right to exist," Sanders said in a 2013 Playboy interview. "In return, the Israelis must end their policy of targeted killings, prevent further Israeli settlements on Palestinian land and prevent the destruction of Palestinian homes, businesses and infrastructure."

That's a stark contrast to the socialist left, which generally sees Israel as a racist, colonial aggressor. Increasingly, leftists advocate a one-state solution to the conflict.

At a town hall meeting during the 2014 Gaza war, Sanders’s sympathy for Israel got him in trouble with some of his constituents. Though Sanders argued that Israel had "overreacted" militarily in Gaza, he also emphasized that Israel was responding to rocket fire coming from heavily populated areas in the Palestinian territory.

When the crowd responded angrily, an infuriated Sanders shouted back: "Excuse me! Shut up! You don’t have the microphone."

That's Bernie on Israel: critical to a degree, but willing to see Israel's side of the issue and staunchly in favor of a two-state solution that is unpopular on the activist left.

"Sanders’ Israel policy seems likely to tilt more toward Tel Aviv than that of Obama," Juan Cole, a historian at the University of Michigan and harsh critic of Israel, writes.

Why does Sanders differ from his base on foreign policy?

The simplest explanation is that challenging the DC foreign policy consensus is really hard. Sen. Rand Paul differs from his party on foreign policy, but has devoted tremendous energy to attempting to pull his party toward him. And it doesn't look like Sanders has the same passion for foreign policy issues.

"While we focus all of our attention on ISIS, the middle class in this country continues to collapse"

That's because foreign policy isn't really Sanders's big thing. As Andrew Prokop's profile makes clear, Sanders's driving passions are economic inequality and the wealthy's stranglehold on politics in the United States. He often tries to refocus discussions of foreign policy onto domestic issue: those global issues shouldn't our primary concern, he suggests, because the real crisis is at home.

"While we focus all of our attention on ISIS, the middle class in this country continues to collapse," Sanders said in a representative CNN interview. "And you know what the people tell me in Vermont and around the country? Let's also start paying attention to the crises facing working families in America."

Those domestic issues are where the bulk of the activist energy on the American left is nowadays. That's where it's likely to remain, perhaps absent any threat of a major new ground war in Iraq or elsewhere. Sanders in a better position, politically, to make a real difference on domestic policy. And that seems to dovetail with his actual views.