WASHINGTON (Reuters) - U.S. President Donald Trump’s withdrawal of forces from northern Syria in October united the Democratic candidates hoping to take on the Republican president in the November 2020 election, who roundly denounced the move as damaging to U.S. credibility.

FILE PHOTO: (L-R) Then U.S. Vice President Joe Biden, Deputy National Security Advisor Tony Blinken, National Security Advisor Susan Rice and Secretary of State John Kerry listen as President Barack Obama and Iraqi Prime Minister Nuri al-Maliki address reporters after their meeting in the Oval Office at the White House in Washington, November 1, 2013. REUTERS/Jonathan Ernst/File Photo

But it also sparked discussion over what they would do in Trump’s position, putting a rare spotlight on how the leading White House hopefuls see America’s role in the world.

On the most recent Democratic debate stage in Ohio this month, former Vice President Joe Biden argued for a continued presence in Syria to defend Washington’ Kurdish allies. His main progressive rival, U.S. Senator Elizabeth Warren, indicated she would want combat troops out of the Middle East.

To understand better how the candidates would change foreign policy, we take a look at the experts advising the front-runners on these issues, and what this little-known group of advisers might mean for a future Democratic administration.

THE VETERAN

Biden’s pick for his senior foreign policy role, Tony Blinken, reflects the claim that former President Barack Obama’s No. 2 is the only Democratic candidate with the experience to undo the impacts of Trump’s presidency.

Blinken, a national security aide in Democratic President Bill Clinton’s White House, has been alongside Biden for years. He advised Biden’s aborted 2008 presidential campaign before joining the vice president on trips to war zones as his national security adviser.

Blinken, who also served as deputy secretary of state during the Obama administration, is seen in the background of the May 2011 photo of Obama’s national security team, which also includes Biden, watching the raid that killed Osama bin Laden in a Pakistan compound.

Adding to the focus on foreign policy experience, Biden has brought in Nicholas Burns, who served as U.S. ambassador to the United Nations in the run-up to the 2003 invasion of Iraq.

Trump this month withdrew U.S. forces from northern Syria, drawing bipartisan rebuke. The president defended the move as part of an effort to “end endless wars.”

In an interview with Reuters, Blinken said that claim was a “con,” since Trump has really put more troops in the Middle East this year. He sent about 3,000 troops to Saudi Arabia in recent months as tensions with Iran escalated.

The Obama administration had worked to limit most U.S. combat deployments to small numbers of troops working with local fighters, Blinken said, but a complete withdrawal from places like Syria could lead to a power vacuum that is “filled by chaos.”

“As much of a burden as it sometimes seems to play this leadership role, the alternatives in terms of our interests and the lives of Americans are much worse,” Blinken said.

Biden says his foreign policy priority would be to recommit to U.S. allies, especially North Atlantic Treaty Organization members. He plans to rejoin the Paris Climate Accord on day one of his presidency.

THE WONKS

U.S. Senator Elizabeth Warren came to prominence as a Harvard academic studying bankruptcy, and has brought a professorial focus on domestic policy to the 2020 race.

But before she ran for president, Warren sought to bolster her foreign policy knowledge in 2017 by bringing in former Department of Defense official Sasha Baker, who served as deputy chief of staff to Obama-era Defense Secretary Ash Carter.

Warren took a seat on the Senate Armed Services Committee in 2017 and traveled widely, including trips to Iraq, Afghanistan and China over the subsequent two years.

Warren formed a “cohesive vision” on foreign policy during this time, said campaign spokeswoman Alexis Krieg, which “is reflected in many of her plans ranging from how to revitalize the State Department to how to reorient America’s trade policies.”

Warren also leans on a cadre of academics for foreign policy advice, including Ganesh Sitaraman, a law professor at Vanderbilt University in Tennessee, who worked on her 2012 campaign for the Senate.

Sitaraman has written about a movement toward a progressive foreign policy that promotes diplomacy over military might, but understands the risks of partnering with “nationalist oligarchies” like Russia and China.

Warren distilled her foreign policy views in a November 2018 speech at American University in Washington, lashing out at “reckless, endless wars in the Middle East” and “trade deals rammed through with callous disregard for our working people”.

Warren’s positions can seem to echo Trump’s “America First” approach, but her campaign has said she would take a multinational approach in places like Syria, a contrast with Trump’s go-it-alone style.

“Senator Warren believes that by pursuing international economic policies that benefit American workers instead of an elite few and using diplomacy to amplify strong yet pragmatic security policies, we can achieve a foreign policy for all,” said Krieg.

THE INSURGENT

Bernie Sanders’ key foreign policy adviser is Matt Duss, a staffer in his Senate office since early 2017 who has a reputation for taking on the Washington foreign policy establishment.

Duss, a Middle East specialist with a background at liberal think-tanks including the Center for American Progress, played a leading role in a resolution that Sanders co-sponsored with libertarian Republicans to end U.S. involvement in Saudi Arabia’s war in Yemen.

The resolution passed in April, a rare bipartisan effort in Congress, but Trump declined to sign it into law.

“That shows that there is strong popular support for rethinking our approach to military intervention,” said Duss, who sees the bipartisan bill as a model for Congress reining in presidents’ power to use military force.

“This is not to say that Americans want to withdraw from the world. They certainly don’t, but they want to have a serious debate about how we actually engage in the world,” Duss told Reuters.

Duss and other researchers Sanders has sought out for advice are skeptical of unconditional U.S. support for countries accused of human rights violations, including Saudi Arabia and Israel.

Sanders, who is Jewish, said on Monday the $3.8 billion of U.S. military aid the United States gives Israel each year should be tied to the Israeli government’s “respect for human rights and democracy,” calling the security blockade on Palestinians in Gaza imposed by Israel and Egypt “absolutely inhumane”

“It is unacceptable. It is unsustainable,” Sanders said in a speech to a conference hosted by liberal advocacy group J Street in Washington on Monday.

“So I would use the leverage. $3.8 billion is a lot of money, and we cannot give it carte blanche to the Israeli government, or for that matter to any government at all.”

THE BRAIN TRUST

Pete Buttigieg, the mayor of South Bend, Indiana, has put foreign policy at the forefront of his campaign, speaking in debates of his deployment in Afghanistan as a naval intelligence officer.

Doug Wilson, a former assistant secretary of defense for public affairs during the Obama administration, leads Buttigieg’s foreign policy team.

Wilson, who like Buttigieg is gay, worked on the 2010 repeal of the military’s “don’t ask, don’t tell” policy for LGBT personnel.

As well as Wilson, Buttigieg has a brain trust of more than 100 foreign policy experts that the campaign turns to for pro-bono advice on policy in different parts of the world, according to Politico.

His campaign has also named a group of five Obama-era U.S. ambassadors, such as Tod Sedgwick, former ambassador to Slovakia, who have helped fundraise by bundling contributions.

At 37, Buttigieg is the youngest candidate running for president. In June, he delivered a foreign policy speech in his home state of Indiana in which he set out a vision for “America in the world in 2054”, the year he would hope to retire.

Buttigieg’s central argument is that the United States must “be its best” on issues like immigration, LGBT rights and tackling hate crimes in order to project its values around the world, Tarek Ghani, an academic and another adviser to Buttigieg, said in Washington in June.

“If we don’t do those things at home, there’s nothing for us to champion abroad,” said Ghani.