He fulfilled his promise.
Without regard to loss of life he pressed on the siege, and though as fast as one line of defences was taken another arose behind it to bar his progress, little by little he advanced and bit by bit the area held by the garrison grew less.

At last in the spring of 1604, under the pressure of the States-General, Maurice led an army of 11,000 men into Flanders in April, 1604, and laid siege to Sluis on May 19.

Both Maurice and William Lewis were still unwilling to run the risk of an attack on Spinola's army in its lines, and so the two sieges went on side by side, as it were independently.

Sluis fell at the end of August, and Ostend was then at its last gasp.

Urged now by the deputies of the States to make a direct effort to relieve the heroic garrison, Maurice and his cousin, after wasting some precious time in protesting against the step, began to march southward.