Complex sentence: Henry had been supported by a number of wealthy landowners to whom he had promised land and money in return for their support and when the war ended, lands in Cumbria promised to the Percys were instead given to a rival.
Split into simpler sentences: Henry had been supported by a number of wealthy landowners to whom he had promised land and money in return for their support. When the war ended lands in Cumbria promised to the Percys were instead given to a rival.