Hector served two violent tours in Iraq as a United States Marine, then got out, got a pension and a civilian job, and thought he was done with military service. But on Friday, he boarded a plane for one more deployment, this time as a volunteer in Ukraine. He checked in several bags filled with rifle scopes, helmets and body armor donated by other veterans.

“Sanctions can help, but sanctions can’t help right now, and people need help right now,” said the former Marine, who lives in Tampa Bay, Fla., and like other veterans interviewed for this article asked that only his first name be used for security reasons. “I can help right now.”

He is one of a surge of American veterans who say they are now preparing to join the fight in Ukraine, emboldened by the invitation of the country’s president, Volodymyr Zelensky, who earlier this week announced he was creating an “international legion” and asked volunteers from around the world to help defend his nation against Russia.