That’s what makes Chafee’s attack intriguing. He was the only Republican senator who voted against authorizing war. Now, as a Democratic candidate for president, he will talk about the collapse of Iraq and the rise of ISIS in a way she can’t. And the way Hillary responds to him will preview her response to the Republicans next fall.

Finding a good answer won’t be easy. On domestic issues like gay marriage, mass incarceration, immigration and campaign finance, Hillary has shifted left in the primary, confident that she’s keeping pace not only with Democratic primary voters but with the country as a whole. But on foreign policy, the public isn’t moving left; it’s moving right. And Hillary cannot as easily adapt her views to match that shift because until two years ago, she was secretary of state.

Hillary’s greatest challenges may not be policy-based at all. They’ll likely stem from questions about her honesty, and the sense that she represents not the future but the past. But if there’s one policy question that could prove dangerous, it’s Iraq. On that subject, Lincoln Chafee is the canary in the coal mine. If Hillary can’t find an effective answer to his attacks, it may be a sign of trouble ahead.