In September 1216 John began a fresh, vigorous attack. He marched from the Cotswolds, feigned an offensive to relieve the besieged Windsor Castle, and attacked eastwards around London to Cambridge to separate the rebel-held areas of Lincolnshire and East Anglia. From there he travelled north to relieve the rebel siege at Lincoln and back east to King's Lynn, probably to order further supplies from the continent.[nb 17] In King's Lynn, John contracted dysentery, which would ultimately prove fatal. Meanwhile, Alexander II invaded northern England again, taking Carlisle in August and then marching south to give homage to Prince Louis for his English possessions; John narrowly missed intercepting Alexander along the way. Tensions between Louis and the English barons began to increase, prompting a wave of desertions, including William Marshal's son William and William Longespée, who both returned to John's faction.

When did John begin a fresh, vigorous attack?