He commanded three hundred foot at the battle of Nieuwport under his brother, after whose retirement from the field he helped Sir John Ogle and Sir Charles Fairfax to rally the English vanguard; and at the Siege of Ostend he took a conspicuous part in the repulse of the Spanish assault on 7 January 1602, being stationed (along with Fairfax) at a vital point in the defenses known as the 'Sandhill,' in command of twelve companies. He was badly hurt in the leg by a splinter. Horace then took to the field under his brother again in the Spring of 1602 with Maurice's army, and after Francis was severely wounded in the head during the siege of Grave he then took over as command of English forces there. Early in April 1603 he was dispatched by his brother with a message to the new king.