MUSCATINE, Iowa — Former Secretary of State John Kerry defended former Vice President Biden over the Iraq War, saying that the George W. Bush administration “broke their word with respect to how they would proceed” in Iraq.

“The fact is that we were promised by a president, by an administration, that they were going to do it as a last resort after exhausting diplomacy, that if they have to go to war it would be with a coalition that they built broadly, and that they would do it only in conjunction with our allies,” Kerry said Friday. “It didn't have to happen; it was a war of choice.”

“It was a mistake to have trusted them, I guess, and we paid a high price for it,” Kerry added. “But that was not voting for the war.”

Senator Bernie Sanders has recently escalated his attacks on Biden over the U.S. invasion of Iraq in 2003.

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“Joe Biden voted and helped lead the effort for the war in Iraq, the most dangerous foreign policy blunder in the modern history of this country,” Sanders said in an interview with CNN earlier this month.

Kerry argued against Sanders’ characterization that Biden voted “for” a war against Iraq.

“I think he knows full well, as a lot of other people do, that there was a difference in people who felt they needed to give a president the leverage to be able to get Saddam Hussein back to the table, without having to go to war, and that that vote was unfortunately structured in a way that it was sort of either-or,” Kerry said. “But that didn't mean you were in favor when the administration made the decision of actually going to war.”

Jeff Weaver, senior adviser on the Sanders campaign, responded Saturday that Biden "tried to distort his record on the Iraq War."

"It is appalling that after 18 years Joe Biden still refuses to admit he was dead wrong on the Iraq War, the worst foreign policy blunder in modern American history," he said in a statement. "Bernie Sanders saw the same information and had the judgment to vote against the Iraq War."

Kerry, acting as a Biden surrogate on a multiday swing through Iowa on a “We Know Joe” bus tour, repeatedly told voters that Biden would be ready as president “on day one.” On the trail, Kerry also frequently praised the former vice president on his foreign policy credentials and international dealmaking.

“The advantage of Joe Biden is that he already has a relationship with these leaders around the world,” Kerry said. “They already know him and trust him. So Joe Biden is the one person who can instantly come into office, begin to undo the chaos and the mess that President Trump has created.”

Kerry was asked about the latest Iowa poll that showed Sanders with a four-point lead over Biden, and he responded, “It's pretty much close.”

“Everybody's jumbled in there all together, and the last weeks are really critical,” Kerry said. “And I think people will see in the debate, and otherwise, a person who's ready to be president, who's got the ideas and the ability to beat Trump, and that's critical. And we’ll see where we are on caucus day.”