Complex sentence: With the defeat and Hamilton and the loss of their two most prominent commanders the Western Association ceased to exist as an effective fighting force removing the threat to Cromwell's flank that had been posed by a force that had consisted of and between 5,000 and 6,000 horse.
Split into simpler sentences: With the defeat and Hamilton and the loss of their two most prominent commanders the Western Association ceased to exist as an effective fighting force. Thus removing the threat to Cromwell's flank that had been posed by a force that had consisted of between 5,000 and 6,000 horse.