under the command of his brother, Louis of Nassau. Valdez lifted the siege in April 1574 to face the invading rebel troops, but Sancho d'Avila reached them first and defeated them in the Battle of Mookerheyde, where Louis was killed. Second siege and relief of Leiden During the brief respite from the siege, Orange counselled the citizens of Leiden to restock their city with supplies, and take in a larger garrison to help defend the town. They disregarded his advice, however, so when Valdez' army returned to renew the siege on 26 May 1574, they were in as poor a condition as they had previously been. The city considered surrendering, as there was almost no chance of relief and supplies were dwindling. The defeat of Louis' army was also a blow to morale. The Prince of Orange, however, was determined to relieve the city. Therefore, he sent a carrier pigeon into the city pleading for it to hold out for three months. To fulfil this promise, he planned to breach the dykes to allow the sea to flood the low-lying land. The siege could then be lifted using the rebel fleet, and the Spaniards would be forced to retire before the incoming sea. This tactic had also been used to relieve Alkmaar. The damage to the surrounding countryside would be enormous, and therefore the population of the area resisted the breaching of the dykes. However, in the end, the Prince prevailed and the outer dykes were broken on 3 August. Previously, the Prince's Admiral Louis Boisot had assembled a fleet of more than two hundred small flat-bottomed vessels, manned by 2,500 veteran Dutch seamen, and carrying a large store of provisions for the starving townspeople of Leiden. Soon after the first dykes were broken, the Prince of Orange came down with a violent fever which brought operations to a halt. More importantly, the flooding of the countryside took longer than expected because of unfavorable winds. On 21 August, the inhabitants of Leiden sent a message to the Prince saying that they had held out for three months, two with food and one without food. The Prince answered them, again by carrier pigeon, that the dykes had all been pierced and relief would come soon. However, only by the first day of September, when the Prince had recovered from his ailment, did the expedition continue in earnest. More than 15 miles