Those men, and some women, have, in general, no combat experience, little training, and few have weapons of their own. They are drawn from the hundreds of thousands of Ukrainians—truck drivers and cabbies, students and IT workers—who have emigrated in search of jobs and prosperity in Europe’s wealthier countries.

“It’s our home, it’s our country, and I don’t have a different one,” said Dmytro, a cryptocurrency trader driving his Volkswagen home to fight…

Polish and other European officials openly support the return of the Ukrainian émigrés, raising the possibility that Europe will become a staging ground for a war that could become a bloody, drawn-out conflict.

“If it was me, I would go too,” said a Polish police officer posted at Poland’s Korczowa border, as cars bringing young and middle-aged men to Ukraine drove past.