Complex sentence: In addition to their own intricate network, the Partisans were provided with intelligence by the British and knew that the Axis would launch an offensive operation around Tito's birthday, but they did not know that it was to be an airborne assault.
Split into simpler sentences: In addition to their own intricate network, the Partisans were provided with intelligence by the British, and knew that the Axis will launch an offensive operation around Tito's birthday. However, they did not know that it was going to be an airborne assault.