It is correct, but a bit too long - it mentions the shortness of his detention twice ('short period of time', 'almost immediately'). Shortening it would involve a bit of rearrangement, it's hard to recommend how, as this needs to fit the overall flow of the paragraph. I'm thinking of something like this: "He was released from prison following the Red Army's conquest of the country, having spent only a short time under arrest". The terms "short [period of] time" and "almost immediately" might mean different things to different people here: an hour under arrest would be a long time for me. Could you be a bit more precise?

Have spent*