Montigny, the son of the Dowager Countess of Hoorn, was one of the envoys sent to Philip's court before the war had actually opened. He had been detained in Spain and feared death, for he was a prisoner in the castle of Segovia. Philip had intended from the beginning to destroy Montigny, but he did not choose to order his execution openly. The knight had been sentenced by the Council of Blood after three years imprisonment, but still lingered on, hoping for release through the exertions of his family. The King was busied with wedding preparations, but not too busy to carry out a crafty scheme by which Montigny seemed to have died of fever, whereas he was strangled in the Castle. The hypocrisy of the Spanish monarch was so complete that he actually ordered suits of mourning for Montigny's servants.

In 1572 the Beggars, always restlessly cruising against their foes on the high seas, took Brill in the absence of a Spanish garrison. Their action was so successful that they hoisted the rebel flag over the little fort and took an oath with the inhabitants to acknowledge the Prince of Orange as their Stadtholder. Brill was an unexpected triumph which the brilliant, impetuous Louis of Nassau followed up by the seizure of Flushing, the key of Zealand, which was the approach to Antwerp. The Sea-Beggars then swarmed over the whole of Walcheren, receiving many recruits in their ranks and pillaging churches recklessly. Middelburg alone remained to the Spanish troops, while the provinces of the North began to look to the Prince of Orange as their legitimate ruler.

William looked askance at the disorderly feats of the Beggars, but the capture of important towns inspired him to fresh efforts. He corresponded with many foreign countries and had his agents everywhere. Sainte Aldgonde was one of the prime movers in these negotiations. He was a poet as well as a soldier, and wrote the stirring national anthem of Wilhelmus van Nassouwen, which is still sung in the Netherlands. Burghers now opened their purses to give money, for they felt that victories must surely follow the capture of Brill and Flushing. William took the field with hired soldiers, and was met by the news of the terrible massacre of Protestants in France in 1572 on the Eve of St Bartholomew. All his hopes of help from France were dashed to the ground at once, and for the moment he was daunted. Louis of Nassau was besieged at Mons by Alva. He tried to relieve his brother, but was ignominiously prevented by the Camisaders who made their way to his camp at night, wearing white shirts over their armour, and killed eight hundred of his soldiers.

William threw in his lot, once for all, with the Northern provinces, receiving a hearty welcome from Holland and Zealand, states both maintaining a gallant struggle. He was recognized as Stadtholder by a meeting of the States in 1572, and liberty of worship was established for Protestants and Catholics. His authority was absolute in this region of the Low Countries.

Alva revenged himself for the resistance of Mons by the brutal sack of Malines and of Zutphen. The outrages of his soldiers were almost inhuman, and immense booty was captured, to the satisfaction of the leader.

Amsterdam was loyal to Philip, but Haarlem was in the hands of Calvinists. The Spanish army advanced on this town expecting to take it at the first assault, but they met with a stubborn resistance. The citizens had in their minds the horror of the sack of Zutphen. They repulsed one assault after another and the siege, begun in December 1572, was turned into a blockade, and still the Spaniards could not enter. The heads of the leaders of relief armies which had been defeated were flung into Haarlem with insulting gibes. The reply to this was a barrel which was sent rolling out carrying eleven heads, ten in payment of the tax of one-tenth hitherto refused to Alva and the eleventh as interest on the sum which had not been paid quite promptly! It was in July 1573, when the citizens had been reduced by famine to the consumption of weeds, shoe-leather, and vermin, that the Spanish army entered Haarlem.

The loss on both sides was enormous, and William had reason to despair. Only 1600 were left of a garrison of 4000. It seemed as if the courage of Haarlem had been unavailing, for gibbets rose on all sides to exhibit the leaders of the desperate resistance.

But the fleets of the Beggars rode the sea in triumph, and the example of Haarlem had given spirit to other towns unwilling to be beaten in endurance. Alva was disappointed to find that immediate submission did not follow. He left the country in 1573, declaring that his health and strength were gone, and he was unwilling to lose his reputation.

Don Luis Requesens, his successor, would have made terms, but William of Orange adhered to certain resolutions. There must be freedom of worship throughout the Netherlands, where all the ancient charters of liberty must be restored and every Spaniard must resign his office. William then declared himself a Calvinist, probably for patriotic reasons.

The hope of assistance from France and England rose again inevitably. Louis of Nassau obtained a large sum of French money and intended to raise troops for the relief of Leyden, which was invested by the Spaniards in 1574. He gathered a force of mixed nationality and no cohesion, and was surprised and killed with his gallant brother Henry. Their loss was a great blow to William, who felt that the responsibilities of the war henceforward rested solely on his shoulders.