The truth is there's no simple answer to that question. The best you can say really is: Well ... maybe.
It's worth noting first, however, that Sanders's comment may have been a slip of the tongue. In the past, the Vermont senator has offered a slightly more cautious assessment of the Iraq war, describing it as being "considered one of the worst foreign policy blunders in the history of the United States." During the debate, his official Twitter account tweeted a message with a similar wording.
(Martin O'Malley, another Democratic presidential hopeful who said he agreed with Sanders on Iraq, also qualified that it was "one of the worst blunders" during the debate.)
Even so, there are certainly those who argue passionately that Iraq really does stand out as the worst U.S. foreign policy decision. In 2013, Peter van Buren, a veteran of the U.S. State Department and writer, wrote a widely circulated post on Tom's Dispatch that put forward that argument on the war's 10th anniversary. As Van Buren put it:
Not to put too fine a point on it, but the invasion of Iraq turned out to be a joke. Not for the Iraqis, of course, and not for American soldiers, and not the ha-ha sort of joke either. And here’s the saddest truth of all: on March 20th as we mark the 10th anniversary of the invasion from hell, we still don’t get it. In case you want to jump to the punch line, though, it’s this: by invading Iraq, the U.S. did more to destabilize the Middle East than we could possibly have imagined at the time. And we — and so many others — will pay the price for it for a long, long time.
Others have, perhaps less enthusiastically, made similar arguments. The Post's own George Will has dubbed the 2003 invasion of Iraq "the worst foreign policy decision in U.S. history." Senate Minority Leader Harry Reid (D) has also called the invasion the "worst foreign policy decision in the history of the country."
It's not too difficult to see how you could come to this assessment. The invasion of Iraq was motivated by the rationale that Iraq possessed weapons of mass destruction. After the invasion, no real evidence of these weapons was found and many suspected U.S. authorities of misleading the public. A relatively quick and successful invasion soon turned into a protracted conflict against an committed and dedicated insurgency. Almost 5,000 U.S. troops died in the conflict, while the total cost of the war was estimated to have been more than $2 trillion in 2013.
By one estimate, the Iraq war resulted in the deaths of at least 144,318 civilians. Many would argue that the conflict created instability in the region and led to further sectarianism and extremism, much like that we see now in Syria.
The thing is, as terrible as the Iraq war was, there have been plenty of other contenders for the worst foreign policy decision in U.S. history. The Vietnam War, for example, is another oft-cited debacle. That war certainly killed more than the conflict in Iraq: Over 60,000 members of the United States armed forces were killed or were missing in action. Total estimates for the deaths from that war range from 1.5 million to 3.8 million.
Another example that would spring to mind for many is Bay of Pigs, the CIA-sponsored military invasion of Cuba in 1961. That invasion is widely remembered as an absolute disaster, with an invasion force of around 1,400 militiamen completely outnumbered by around 200,000 to 300,000 forces loyal to Fidel Castro's left-wing government. As historian Theodore Draper famously put it, it was "one of those rare events in history — a perfect failure." It helped contribute to a Cold War antagonism that could potentially have descended into nuclear war.
Other suggestions might be the CIA's involvement in the 1953 Iranian coup that ousted the democratically elected government of Mohammad Mossadegh, a move that seriously damaged the U.S. reputation in the Middle East and directly informs the antagonism between Washington and Tehran that continues to this day. Or an argument could be made for the U.S. failure to join the League of Nations after it was created in 1919, a decision that many argue contributed greatly in the organization's failure to bring world peace.
Your perspective on which foreign policy blunder was the worst likely depends on a variety of factors, including (but not limited to) ideology and attitudes to history: At the end of the day, it's likely subjective.
What we can say is that, polls do suggest that most Americans view the Iraq war as a mistake. Last year, for example, the polling company YouGov asked what was the biggest mistake in American history. The Iraq and Vietnam wars came in tied at third place, each with 8 percent of the vote. The only mistakes that placed higher were the treatment of Native Americans (with 12 percent) and slavery (with 30 percent) — mistakes that might now be considered domestic issues, though in the past had considerable elements of foreign policy.