Complex sentence: The first years of the Great Northern War did not affect Pomerania and even when Danish, Russian and Polish forces had crossed the borders in 1714, Prussia first appeared as a hesitant mediator before turning into an aggressor.
Split into simpler sentences: The first years of the Great Northern War did not affect Pomerania. Even when Danish, Russian, and Polish forces had crossed the borders in 1714, the Kingdom of Prussia first appeared as a hesitant mediator before turning into an aggressor.