Democratic House candidate Seth Moulton, right, defeated incumbent John Tierney Tuesday. (AP Photo/Jim Davis, The Boston Globe)

Democrat Seth Moulton defeated Rep. John Tierney (D-Mass.) Tuesday, surging late in the primary and beating back a raft of Democratic establishment support for the incumbent.

So, just who is Moulton? Here are the six most important things to know about him:

1. He's an Iraq war veteran who opposed the war. 

After graduating from Harvard , Moulton joined the Marines in 2001. He served in Iraq and was part of the first Marine company to enter Baghdad, despite opposing the war, according to his campaign biography. "While I believe that the war was a mistake, I don't regret doing what I could to serve our country," Moulton said in his introductory campaign video. These days, with Iraq back at the center of the national conversation amid the rise of the Islamic State, Moulton opposes a new ground war there.

2. His parents were Vietnam war protesters. 

Boston Magazine has the details:

As students at Brown University in the late 1960s, Lynn and Tom opposed the Vietnam War. During her senior year, Lynn joined students at campuses across the country who, as an act of protest, refused to take their final exams. Later, as a law school student at Boston University, Tom marched in an antiwar demonstration. After marrying and starting a family, they wanted to raise Seth and his younger siblings, Cyrus and Eliza, to be independent thinkers. "They had a teacher who preached leftist values," says Lynn, a gregarious woman who wears glasses and her hair cropped short. "I wanted to say, ‘I agree with you, but I don’t want you to tell them what to think.’"

As parents, Lynn and Tom’s own open-mindedness had its limits. While the children were growing up, Lynn prohibited them from playing with toy weapons, even squirt guns. ("I gave them plastic fish to squirt at one another," she says.) When Seth informed his parents that he planned to enlist in the Marines, Lynn says, her first thought was: "There was no career choice he could have made that would have made me more unhappy, except if he had chosen a life of crime."

3. He's backed by Stanley McChrystal. 

The retired Army general, who was dismissed as the top commander in Afghanistan in 2010, bestowed his first political endorsement on Moulton.

4. He probably gives Democrats a better chance at holding the 6th district. 

Tierney was a lackluster campaigner who was still carrying baggage from his family gambling scandal. Democrats are probably better off with Moulton as their nominee. Republican nominee Richard Tisei, who narrowly lost to Tierney in 2012, would have been able to tie the congressman to the Obama administration and hammer his long stay in Washington at a time when voter fatigue with Congress is rampant. With Moulton, he's running against a fresh face in a district that handed Obama 55 percent of the vote in 2012.

5. That said, Republicans will try to cast him as a Tierney clone.

"Seth Moulton may have defeated John Tierney in the primary, but if elected to Congress Moulton would be a less effective carbon copy [than] the inept Tierney. As Moulton said, he agrees with Tierney ‘on all the issues’ and that is exactly what makes him so wrong for Massachusetts families and small businesses," said National Republican Congressional Committee spokesman Ian Prior in a statement.

 6. He's young.

Moulton is just 35 -- which, if he's elected, would make him one of the youngest members of the next Congress.