The siege which ensued was one of the memorable sieges of history, it lasted for more than three years (July 15,1601, to September 20,1604) and was productive of great feats of valour, skill and endurance on the part alike of besiegers and besieged.

The States' army under Maurice, though it did not march to the relief of Ostend, endeavoured to divert the attention of Albert from his objective by attacks directed elsewhere.

In 1601 the fortresses of Rheinberg and Meurs on the Rhine were captured, and an attack made upon Hertogenbosch.

In 1602 the important town of Grave on the Meuse was taken and a raid made into Brabant and Luxemburg.
Meanwhile the defenders of Ostend had been making a desperate resistance, and little progress was made by the besiegers, with the result that a great drain was made upon the finances of the archdukes and there were threatenings of mutiny among the troops.

But the situation was saved by the intervention of a wealthy Genoese banker, Ambrosio de Spinola, who offered his services and his money to the archdukes and promised that if he, though inexperienced in warfare, were given the command, he would take Ostend.