Negociations for peace however were still being carried on in 1587 between Spanish and English plenipotentiaries. It was mainly the merchants of London and Antwerp that urged it; and as the Spaniards at that time had manifestly the best of the struggle, were masters of the lower Rhine and the Meuse, had invaded Friesland, had besieged and at last taken Sluys in despite of all resistance, we can understand how the English plenipotentiaries were moved to unexpected concessions. They would have consented to the restoration of the Spanish supremacy over the northern Netherlands, if Philip would have granted the inhabitants freedom of conscience. Alexander of Parma brought forward a proposal, to make, it is true, their return to Catholicism obligatory, but with the assurance that no Inquisition should be set over them, nor any one punished for his deviation from the faith. Even if the negociation was not meant to be completely in earnest, it is worth remarking on what rock it was wrecked. Philip II would neither grant such an assurance, which in its essence involved freedom of conscience, nor grant this itself completely in a better form. His strength lay precisely in his maintaining the Catholic system with unrelenting energy: by this he secured the attachment of the priests and the zealous laity. And how could he, at a moment when he was so closely united with the Pope, and could reckon on the millions heaped up in the castle of St. Angelo for his enterprise, so completely deviate from the strictness of exclusive belief. He thought he was within his right when he refused any religious concession, seeing that every other sovereign issued laws prescribing the religion of his own territories.[268] If the war was to be continued, Alexander of Parma would have wished that all his efforts should be first directed against Vliessingen, where there was an English garrison; from the harbour there England itself could be attacked far more easily and safely. But it was replied in Spain that this enterprise was likewise very extensive and costly, while it would bring about no decisive result. And yet Alexander himself too held an invasion of England to be absolutely necessary; his reports largely contributed to strengthen the King in this idea; Philip decided to proceed without further delay to the enterprise that was needful at the moment and opened world-wide prospects for the future.