On paper, there are only two armies lobbing artillery shells and ammunition at each other: Russia and Ukraine.

But the confusion about what constitutes a country joining the fight deepened this week, after the U.K. military reported that a 19-year-old serviceman had gone AWOL from his post in the Coldstream Guards—an elite unit that services Queen Elizabeth at Windsor Castle—and was apparently headed to fight in Ukraine.

British tabloid newspaper The Sun reported that the soldier, whose name is being withheld, was one of four active military servicemen who had absconded to go fight in Ukraine. According to the paper, he wrote to his parents before he left, telling them that he was buying a one-way ticket to Poland, the main entry point into Ukraine, whose airports have been shut since Russia invaded the country on Feb. 24.

Ukraine’s besieged government formed an international legion on Feb. 27, appealing for volunteers to bolster their military strength against the far bigger Russian forces. “Together we defeated Hitler, and we will defeat Putin too,” tweeted Foreign Minister Dmytro Kuleba, in announcing the foreign-fighters unit.

The foreign minister said on the weekend that about 20,000 volunteers from 52 countries have signed up, and Ukraine’s embassies report they have seen a steady stream of people seeking documentation allowing them to travel to the war front to join the fight.

From parliament to the war front

Facebook groups have sprung up to guide prospective fighters through the process of signing up; one Facebook group, the French Volunteers in Ukraine group, now has 10,700 members. And on Wednesday, Latvia announced that its Member of Parliament Juris Jurašs had left to fight in Ukraine; the Baltic nation shares a 133-mile border with Russia.

Until now, Western countries have stopped short of joining Ukraine’s military fight, focusing their efforts on imposing economic sanctions, and believing that engaging military would transform the conflict into a far broader war in Europe—something that has not occurred in 80 years—from which it could take years to extricate.

The fact that active U.K. servicemen might be among them has sparked deep anxiety in the government. It has banned active-duty military from joining Ukraine’s volunteers.

A friend of the missing Queen’s guardsman told The Sun that the serviceman had grown tired of ceremonial duties at Windsor Castle, where the guards march in formation outside the castle, dressed in ornate uniforms topped with their famous bearskin hats. “You don’t join the army to stand in bearskin hats and march about,” the guardsman told the paper. “You join it to fight and see action.”

The U.K. Ministry of Defense repeated its warning against servicemen joining the fight in Ukraine, saying they would face “disciplinary and administrative consequences.”