Excerpts from Books and Wikipedia
"However, the accession of the unpopular John to the English throne on the death of Richard I in 1199 gave Philip his chance. Cunningly exploiting a twist of feudal law that in some circumstances made John subject to his jurisdiction, Philip summoned him in 1201 to defend the contested legality of his marriage to Isabelle of Angouleme. John understandably declined to attend. Philip responded by declaring most of John's French lands forfeit. . . . .Philip invaded Normandy with 6,000 men and took the English fortress of Chateau-Gaillard after a long siege. The rest of Normandy succumbed without a fight. Rouen, however, had strong trading links with England. . . . After a forty-day siege, with no help from John forthcoming, Rouen surrendered on June 24." [Furtado: 1001 Days]