In February, Louis set sail for France to gather reinforcements. In his absence, arguments broke out between Louis's French and English followers, and Cardinal Guala declared that Henry's war against the rebels was a religious crusade. This resulted in a series of defections from the rebel movement, and the tide of the conflict swung in Henry's favour. Louis returned at the end of April and reinvigorated his campaign, splitting his forces into two groups, sending one north to besiege Lincoln Castle and keeping one in the south to capture Dover Castle. When he learnt that Louis had divided his army, William Marshal gambled on defeating the rebels in a single battle. William marched north and attacked Lincoln on 20 May; entering through a side gate, he took the city in a sequence of fierce street battles and sacked the buildings. Large numbers of senior rebels were captured, and historian David Carpenter considers the battle to be "one of the most decisive in English history".