Mr. Weston, who is now writing a book but remains in close contact with scores of the men he served with, said Marines across the globe had been frenetically sharing their feelings about the new battle for Falluja via email, text and Facebook.

“The news went viral in the worst way,” he said. “This has been a gut punch to the morale of the Marine Corps and painful for a lot of families who are saying, ‘I thought my son died for a reason.’ ”

Ryan Sparks was a platoon commander during a seven-month Falluja deployment in which three men were killed and 57 wounded in his 90-man unit. Now about to take a job in Manhattan after recently leaving the Marines, Mr. Sparks, 39, said many of the younger Falluja veterans are angry “because we lost so many Marines, and it feels like they were sacrificed for nothing.”

Yet even among older officers who seem less surprised by the turn of events, Mr. Sparks said, “It hurts to think that it isn’t as important to Americans as it was to us while it was happening.”

He likens Falluja to Khe Sanh, the bloody 1968 battle where Americans triumphed only to abandon the base months later, though he did not disagree with the 2011 troop pullout and does not believe that American troops should be sent back in.

“This makes the analogy complete,” he said.

Mr. Banotai has no regrets about supporting the war, and said it was a mistake for the United States to withdraw troops when it did, which he believes was done for political reasons, not because the mission was accomplished. But he also would not favor sending troops back. “It’s too late. Mistakes have already been made,” he said. “We can’t go back and rewrite history.”