Complex sentence: Happening during the timespan of the Great Purge, fascism in Italy and Germany had greatly risen up, seeing this as a potential threat the Party actively sought to form "collective security" alliances with Anti-fascist western powers such as France and Britain.
Split into simpler sentences: Happening during the timespan of the Great Purge, fascism had ascened to power in Italy and Germany. Seeing this as a potential threat, the Party actively sought to form "collective security" alliances with Anti-fascist western powers such as France and Britain.