In the case of the Spanish civil war, the problem was that the communists and the fascists both wanted them killed, and so with some effort they succeeded.
Loads and loads of books, historical documents, documentaries , and so on on the matter.
Not just in Spain, in USSR and elsewhere too.
In fact the ideological enmity between communists and anarchists starts even back at the time of Bakhunin and Marx (and they were many critiques from one to another).
The strategy of the Stalin-backed communists was to not alienate the middle class. The anarchists, on the other hand, were not about letting war get in the way if social reform, and had taken over factories and collectivised land. It was also a good opportunity for Stalin to purge communists not allied with the USSR.
