On May 26 a strong force under Valdez advanced to Leyden and completely isolated the town by surrounding it with a girdle of forts.
The attack came suddenly, and unfortunately the place had not been adequately provisioned.

So strong was the position of the Spaniards that the stadholder did not feel that any relieving force that he could send would have any chance of breaking through the investing lines and revictualling the garrison.

In these circumstances he summoned, June 1, a meeting of the Estates of Holland at Rotterdam and proposed, as a desperate resource, that the dykes should be cut and the land submerged, and that the light vessels of the Sea-Beggars under Boisot should sail over the waters, attack the Spanish forts and force an entrance into the town.

After considerable opposition the proposal was agreed to and the waters were allowed to flow out upon the low-lying fields, villages and farms, which lie between the sea, the Rhine, the Waal and the Maas.
Unfortunately the season was not favourable, and though the water reached nearly to the higher land round Leyden on which the Spanish redoubts were erected, and by alarming Valdez caused him to press the blockade more closely, it was not deep enough even for the light-draught vessels, which Boisot had gathered together, to make their way to the town.

So the month of August passed and September began.