In May 1604 he was at the siege of Sluis, commanding troops which routed the Spanish general Luis de Velasco in the battle of Oostberg line. According to Dutch military historians François de Bas and F.J.G. Ten Raa, Fairfax later became commander of the English companies still fighting at Ostend. He landed there on 7 June with five companies of reinforcements, but was killed in the last stages of fighting on 17 September, only three days before the general surrender of the stronghold.