Cecil served with the English forces in the Netherlands between 1596 and 1610, becoming a captain of foot in 1599. In May 1600 he was appointed to a troop of cavalry, which he commanded at the battle of Nieuport, under Sir Francis Vere. In 1601 he commanded a body of one thousand men raised in London for the relief of Ostend, then besieged by the Spanish, and on his return in September was knighted by Queen Elizabeth. He was elected Member of Parliament for Aldborough in 1601. In the spring of 1602 he was colonel of a regiment of English horse under Prince Maurice, and served in the expedition into Brabant and at the Siege of Grave. He commanded the English cavalry during the conquest of Sluis in 1604 during which he played a part in defeating the relief attempt by Ambrogio Spinola. He continued actively serving during the years immediately following, and made his reputation as a soldier. In 1610 he commanded the British and Irish contingent of four thousand men serving under Prince Christian of Anhalt in the War of the Jülich succession, at the siege of Juliers in July and August. This was the first ever army flagged as British under the Union Flag.