Baronial unrest in England prevented the departure of the planned 1205 expedition, and only a smaller force under William Longespée deployed to Poitou. In 1206 John departed for Poitou himself, but was forced to divert south to counter a threat to Gascony from Alfonso VIII of Castile. After a successful campaign against Alfonso, John headed north again, taking the city of Angers. Philip moved south to meet John; the year's campaigning ended in stalemate and a two-year truce was made between the two rulers.
Why did John divert south?
to counter a threat to Gascony from Alfonso VIII of Castile