On Sunday, CBS News released a poll of Democratic voters in Iowa showing a three-way tie between Bernie Sanders, Joe Biden, and Pete Buttigieg, all having 23 percent support in the upcoming primary. Beyond this top line, which won all the headlines, there were fascinating nuggets inside the poll. One question was what topic did voters want to see the candidates discuss. The rundown was health care (39 percent), climate change (20 percent), jobs and the economy (18 percent), gun policy (8 percent), immigration (6 percent), judges and the courts (4 percent), race and gender (3 percent), and abortion (1 percent).

What was noticeably missing from the list was any foreign policy issue. This is partially the fault of the pollster, but not exclusively. After all, on the campaign trail, the candidates themselves have mostly been talking about domestic issues. Medicare for All and the Green New Deal have been the policies dividing Democrats, not the endless wars in the Middle East or the future of America’s relationship with China. The poll merely reflected the priorities candidates have already articulated. Yet the fact that foreign policy gets only cursory consideration is strange, because this is one area where an American president has enormous power to make lasting changes.

In theory, Friday’s assassination of Iranian military leader Qassim Suleimani should move foreign policy front and center. It was a reckless decision, an act of war that will almost certainly accelerate the cycle of violence between Iran and the United States. All the Democratic candidates have condemned Trump’s act, but have done so in differing terms, which should open the door for a robust foreign policy skirmish.

Writing in The Intercept, Robert Mackey notes, “Senators Bernie Sanders and Elizabeth Warren described the drone strike ordered by Trump as a dangerous escalation and promised to end American wars in the Middle East. Joe Biden, the former vice president, and Pete Buttigieg, the former mayor of South Bend, Indiana, offered more muted criticism, suggesting that the killing of Maj. Gen. Qassim Suleimani might have been justified if a more responsible commander-in-chief was in charge.”

Warren initially hedged in her comments, hinting that the assassination might have been justified in other circumstances. But, after being criticized by progressives, she quickly found her feet on the issue and now articulates a forthright position against a new war with Iran.

Matt Duss, Sanders’s chief foreign policy adviser, used the issue to make a critique of Sanders’s rivals. Duss tweeted that saying, “Yeah but what’s your strategy?” isn’t a brave stance but is “the safest, lamest, most DC criticism possible. Trump’s Iran policy is foolish, dangerous, and driven by hardline ideologues who’ve been pushing for war for years. Who cares what the strategy is for achieving it. The policy needs to change.”

Sanders is the candidate best positioned to use the ramping up of hostilities in the Middle East as an opportunity to highlight his alternative policy. He’s had a long record as an anti-war activist, going back to the Vietnam War. In 2016, he gained some traction by contrasting his vote against the Iraq War with Hillary Clinton’s vote in favor. In the current race, Sanders has staked out a radical foreign policy that includes ending the forever wars, using aid money as leverage to push Israel to negotiate with the Palestinians, and making climate change a cornerstone of American foreign policy.

In a tweet, Sanders boasted:

I was right about Vietnam.   I was right about Iraq.   I will do everything in my power to prevent a war with Iran.   I apologize to no one.

The strategic question facing the Sanders campaign is how strongly to lean into the anti-war position. The advantages are that it would allow Sanders to make a clear contrast with his moderate rivals and run on an impressive record. The downside is that it might dilute the very popular domestic agenda, particularly Medicare for All, that has defined Sanders’s run.

Reporting from Iowa, David Weigel of The Washington Post finds little evidence that Sanders’s anti-war stance is resonating. “In the first 48 hours after the killing, many voters said they were nervous about the risk of a wider war but not following every detail of the story,” Weigel observes. “Rather than sparking a debate among Democrats, President Trump was giving them another reason that just one action—replacing him with one of the Democratic candidates—would let them stop worrying all the time.”

For at least some of the Iowa voters Weigel talked to, the flare-up of tensions with Iran plays to Joe Biden’s strengths. After all, Biden has foreign policy experience in the Obama administration, which most Democrats remember fondly. Even those who might question Obama’s decision to intensify drone attacks and maintain a military presence in Iraq and Afghanistan still see the former president as superior to Trump’s chaotic belligerence.

The difficulty for Sanders is that if he raises the salience of foreign policy, he’ll have to make an argument against not only Trump but also Barack Obama. It’s not enough to just slag Trump when the general consensus is that any of the Democratic candidates would be superior to Trump. The special case that Sanders is making is Democrats have to break with some of Obama’s signature policies. The argument Sanders would have to put forth is that returning to Obama’s policies, which is what Biden is running on, is dangerous and untenable.

The safer course for Sanders would be to stick to domestic policy, where his popular policies distinguish him enough that they could carry him to victory. But Trump’s recklessness might make that safer path impossible. The United States is already in a de facto war with Iran. If that war becomes a large-scale conflict, Sanders will have no choice but to run as the only true anti-war candidate.