Foreign policy played a major role in the first debate featuring the Democratic presidential candidates on Tuesday night, with the hopefuls sparring with frontrunner Hillary Clinton over Iraq, Syria and the use of military force.

Clinton, the former secretary of state, reiterated her support for an American-led no-fly zone over parts of Syria to stabilize the region, an option opposed by the current White House and supported by a number of Republican leaders.

The issue became the largest sparring point for the candidates during the debate, an animated but largely more calm and civil event than the 10-plus-candidate Republican debates earlier this year.

Independent Vermont Sen. Bernie Sanders, whose unexpected rise in the polls has chipped away at Clinton's popularity among Democrats, said the no-fly zone idea in Syria would be dangerous for U.S. pilots and could potentially draw America into another lengthy, costly war.

"As (former) chairman of the Senate Veterans' Affairs Committee, I learned powerful lessons about the costs of war," he said. "I will do all I can to make sure we don't get involved in another Iraq."

Former Rhode Island Gov. Lincoln Chafee also repeatedly invoked the Iraq War during the event, calling it the cause of the current chaos in the Middle East. He and former Maryland Gov. Martin O'Malley criticized Clinton for her 2002 vote authorizing that war, saying it calls into question her judgement as a potential commander in chief.

When asked to name the nation's top security threat, Clinton and O'Malley mentioned the spread of nuclear weapons. Chafee cited unrest in the Middle East. Sanders said the biggest worry should be climate change, and the secondary destabilizing effects it can bring.

Former Virginia Sen. Jim Webb used the question to call out China, saying that nation's moves in the Pacific present a direct challenge to U.S. superiority in the region.

He also labeled the Obama administration's negotiations with Iran as "the deal that gives them a nuclear weapon" and said that his experience as a Vietnam veteran and an assistant secretary of defense gives him the strongest resume to lead the country.

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None of the candidates named Russia as the top national security threat, despite Pentagon service chiefs calling recent regional aggression by Russian president Vladimir Putin their biggest concern. Clinton did say of Putin that America needs to "stand up to his bullying" and that Russian involvement in Syria cannot go unchecked.

The Democratic candidates did not mention sequestration or military personnel cuts, which were brought up in the earlier Republican debates.

But both Sanders and Webb discussed veterans issues, which were absent from those GOP events. Webb mentioned his work on the post-9/11 GI Bill while in Congress and his service with the Marines during the Vietnam War.

Sanders was asked if he was prepared to serve as commander in chief as a conscientious objector in that war, and responded by saying that he is not a pacifist but "wars should be the last resort."

He was also questioned about his oversight of the Veterans Affairs Department during his time as chairman of the Senate Veterans' Affairs Committee. Sanders said he worked on reforms aimed at adding more doctors and appointments to VA services, but dodged questions about signs that lawmakers missed leading up to the department's care scandals.

Tuesday's event was the first of six planned debates among the Democratic candidates. The Republicans have scheduled almost twice as many; their next one will be held Oct. 28.