There are examples of anarchists fighting by themselves, most notably the Free Territory of Ukraine (for the most part, though it did fight alongside the red army on multiple occasions) there's also the AANES which isn't inherently anarchist but does have a lot of it's libertarian characteristics, there's the zapatistas which you could argue that they are anarchists and I'd say they're definitely fighting alone, and of course Revolutionary Catalonia, which created a (functional?) society while still facing the much stronger fascists to the west and a lot of internal repercussion from republicans/marxists.

This is also ignoring the fact that most anarchists would rather fight for a lesser evil when possible instead of sitting around doing nothing.