U.S. veterans are training Ukrainians near the front lines in the fight against Russia.

A democracy came under attack. The United States saw a threat to an ally and also to the entire world order, but it feared that sending troops could incite a nuclear war. So, instead, it supplied weapons. And a small number of American Special Operations trainers started quietly working with the local military.

That was the situation in South Vietnam in 1961, a few years before full-blown U.S. military involvement, when the American presence was limited to a military “advisory group.”

It is also the situation in Ukraine today. As a bloody conflict churns on, small teams of American Special Operations veterans are training Ukrainian soldiers near the front lines and, in some cases, helping to plan combat missions.

There is a notable difference, though. In Vietnam, the trainers were active-duty troops under the control of the Pentagon. In Ukraine, where the United States has avoided sending any troops, the trainers are civilian volunteers, supported by online donations and operating entirely on their own.

“This is why I became a Green Beret,” said Perry Blackburn Jr., a retired Army Special Forces lieutenant colonel who spent 34 years in uniform in Iraq, Afghanistan, Ethiopia, Egypt, Somalia and Jordan. He is now in Ukraine as a civilian doing what he once did in the military: training local forces to fight a common enemy.

“To not use my talents in a real time of need would be a waste,” said Mr. Blackburn, 60, who was one of a handful of Special Forces soldiers who rode into Afghanistan on horseback at the start of the U.S. invasion in 2001 and is funding similar efforts now through thousands of small online donations from the public.

“At my age, I’ve seen enough death and I want to try to stop the bloodshed,” he said. “We need to give people the means to defend themselves.”

Whether this new type of crowdfunded military support is wise is up for debate.