The statement comes in the wake of the Vermont senator's call at Sunday's debate to normalize relations with the longtime U.S. foe.
"The stakes are high. And we are concerned that Sen. Sanders has not thought through these crucial national security issues that can have profound consequences for our security," the diplomats said. "His lack of a strategy for defeating ISIS -- one of the greatest challenges we face today -- is troubling. And the limited things he has said on ISIS are also troubling."
Among those who signed the statement are Ambassador Wendy Sherman, the under secretary of state for political affairs who led the nuclear negotiations with Iran; Jeremy B. Bash, former chief of staff to the director of the CIA and Ambassador Nicholas Burns, former under secretary of state for political affairs.
Michael Briggs, a Sanders campaign spokesman, said that while Clinton has more experience, Sanders' "judgment on major foreign policy issues is far superior."
At Sunday's debate, Sanders said it was time for the United States to "move as aggressively as we can to normalize relations with Iran, understanding that Iran's behavior in so many ways in something that we disagree with."
"Their support for terrorism, the anti-American rhetoric that we're hearing from some of their leadership is something that is unacceptable," Sanders said. "On the other hand, the fact that we managed to reach an agreement, something that I very strongly supported, that prevents Iran from getting a nuclear weapon and that we did that without going to war and that I believe we're seeing a thaw in our relations with Iran is a very positive step."
Sanders said he doesn't think the United States should open an embassy in Tehran "tomorrow," but did liken normalizing relations with Iran to the move the United State has made with Cuba.
Top White House aides, and President Barack Obama himself, have said that normalizing relations with Iran is not happening anytime soon.
"Will we seek to gain more cooperation from them in resolving issues like Syria, or what's happening in Iraq, to stop encouraging Houthis in Yemen? We'll continue to engage with them," Obama said in July
. "Although, keep in mind that unlike the Cuba situation, we're not normalizing diplomatic relations here. So the contacts will continue to be limited."
Ben Rhodes, Obama's top national security adviser, also said this month that "the U.S. will not also not normalize relations with Iran after the landmark nuclear deal is fully implemented."
The group of diplomats, some of whom worked for Clinton at the State Department, also said Sanders' call for more Iranian involvement in Syria as "dangerous and misguided and the opposite of what is needed."
"Given these concerns, it is important to ask what he would do on other issues -- on Russia, China, our allies, nuclear proliferation and so much else. We look forward to hearing him address these issues," the group said in the statement. "We need a commander in chief who sees how all of these dynamics fit together -- someone who sees the whole chessboard, as Hillary Clinton does."
Sanders, who has risen in the polls and is now directly challenging Clinton in Iowa and leading in New Hampshire, has captured the attention of many liberals, in large part, because of his focus on economic and domestic issues, not foreign policy.
After terrorist attacks in December turned the focus of the broader campaign to security, Sanders defended framing questions about foreign policy, terrorism and other issues as a byproduct of economic inequality. In response, he has begun including ISIS in every stump speech.
The list of diplomats in Tuesday's statement include:
- Rand Beers, former deputy homeland security adviser to the president of the United States; former acting secretary of the U.S. Department of Homeland Security
- Ambassador Daniel Benjamin, former U.S. ambassador-at-large and coordinator for counterterrorism at the State Department
- Derek Chollet, former assistant secretary of defense for international security affairs
- Kathleen H. Hicks, former principal deputy under secretary of defense for policy
- Lt. General Donald Kerrick (Ret.), former deputy national security adviser to the president of the United States
- James N. Miller, former under secretary of defense for policy
- Julie Smith, former deputy national security adviser to the vice president of the United States