King Philip II conceived a plan which may have worked had all of its parts come together. Alexander Farnese, the Duke of Parma, was a key part of the operation's success.
When the Armada reached Dunkirk, Parma was to provide lighter, faster ships (in contrast to the slow-moving Spanish galleons) and an Army (to help invade, and conquer, Britain):
The Spaniards forthwith, as they had done many times before, urged the Duke of Parma, by messengers dispatched one after another, to send 40 fleiboates, that is, light vessels, without which they could not well fight with the English by reason of the over greatnesse and slownesse of the Spanish shippes and the singular agility of the English; and they most earnestly prayed him to put to sea with his Army, which the Spanish fleete would protect as it were under her wings (so it was resolved) till they were landed in England.
For various reasons, however, Parma did not fulfil his part of the mission:
But hee being unready could not bee present at their call, his flatt-bottomed boates for the shallow channells leaked, his provision of victualls was not ready, and his sailers, having beene stayed hitherto against their wills, had withdrawne themselves. (Camden, Annales Rerum Angliae et Hiberniae Regnante Elizabetha, 1588, Section 25)
Parma had other problems. At the time, Spain was an occupying power in Holland (The Netherlands). Dutch forces were rebelling against the intrusion. Helping Britain was a way to help themselves:
There lay watching also at the entrance of the havens [harbors] of Dunkirke and Nieuport, whence he [Parma]was to put to sea, the ships of warre [war] of the Hollanders and Zelanders, so strongely provided of great ordnance and musketiers that hee could not put from shoare, unlesse he would wilfully thrust himselfe and his upon present death. And yet he, a skilfull and industrious warriour, seemed to omit nothing, being inflamed with desire of the conquest of England. (Camden, Annales Rerum Angliae et Hiberniae Regnante Elizabetha, 1588, Section 25)
And then there was the wind. Unseasonably strong, and usually coming from the wrong direction, bad winds dogged the Spanish fleet as the great ships slowly fought their way through the English Channel.
How could King Philip have anticipated that the wind would become such a significant player in the coming battles?