Complex sentence: He established an anti-communist organization along with Jaromír Nechanský which collaborated with the OSS, particularly the US secretary Walter Birge posted to the embassy in Prague, and for this he was arrested in 1949 and sentenced to death after a trial.
Split into simpler sentences: He established an anti-communist organization along with which collaborated with the Office of Strategic Services, particularly the American secretary Walter Birge who was posted to the Prague embassy. For this he was arrested in 1949 and sentenced to death after a trial.