Complex sentence: Piero Strozzi, commanding the Siense forces, won at Pontedera but this only led to his decisive defeat two months later at the Battle of Marciano and the end of the Senese independence.
Split into simpler sentences: Piero Strozzi, commanding the Siense forces, won at Pontedera. Two months later he was decisively defeated at the Battle of Marciano, an event which marked the end of the Senese independence.