The war slowly turned against Philip over the course of the next three years. Political and military conditions seemed promising at the start of 1196 when Richard's nephew Arthur I, Duke of Brittany ended up in Philip's hands, and he won the Siege of Aumale, but Philip's good fortune did not last. Richard won over a key ally, Baldwin of Flanders, in 1197. Then, in 1198, Holy Roman Emperor Henry VI died. His successor was to be Otto IV, Richard's nephew, who put additional pressure on Philip. Finally, many Norman lords were switching sides and returning to Richard's camp. This was the state of affairs when Philip launched his campaign of 1198 with an attack on Vexin. He was pushed back and then had to deal with the Flemish invasion of Artois.