Complex sentence: When the Swedes were reinforced on 1 April it was decided that they would attempt to break the siege, this was done with some success since the Swedes managed to take Usedom and Wolin.
Split into simpler sentences: When the Swedes were reinforced on 1 April it was decided that they would attempt to break the siege. This was done with some success since the Swedes managed to take Usedom and Wolin.