The alliance with France was in the meantime maturing. There was no longer any reason for delaying an open declaration of hostilities against Spain. On the contrary, as Penn and Venables had had time to develop their attack on the West Indies, it was very desirable that the Spaniards should be prevented from sending reinforcements, and this could be most effectually done by compelling them to stand on guard at home. Blake was therefore ordered to cruise off Cadiz, for the double purpose of intercepting the treasure-ships on their way back from America and of preventing the despatch of a Spanish squadron to the West Indies. The blockade was so far successful that the Spaniards were paralysed, but no prizes were taken, and by the approach of autumn the English ships were severely strained. In October Blake returned to England. The war with Spain had not as yet been prosecuted with very triumphant success. The failure at San Domingo was a huge disgrace, hardly balanced in the opinion of the world by the capture of Jamaica. The successful blockade of Cadiz, though it must have caused great loss to the enemy, had not produced those visible results in the way of prizes and bullion which the nation could understand. But Cromwell was resolved to persevere.
In 1656 Blake returned to the coast of Spain. On this cruise he was accompanied by Edward Montagu, afterwards the first Earl of Sandwich, and their object was again to capture the much-desired Plate Fleet. This year they had a better chance of success. In 1655 the Spanish Government had stopped the vessels, but it was now in such dire need of money that it was compelled to run the risk of bringing them home. In summer a first detachment reached the neighbourhood of Cadiz, only to fall into the hands of the English blockading fleet. Blake and Montagu were not present at the capture, for they had retired to the friendly ports of Portugal to refit, but a squadron had been left to watch the port, under the command of Richard Stayner. It proved amply sufficient for the work to be done. The Spanish treasure-ships, though of great bulk for the time, were intrinsically very feeble, and their decks were hampered by merchandise. Stayner burned or captured nearly the whole, with a very trifling loss to himself. The bullion and goods taken amounted in value to nearly the revenue of England for a year--to over two millions sterling. Montagu returned to England with the booty, taking Stayner with him. Although the great sugar-loaves of silver were pillaged by the sailors, there was enough left to load thirty waggons, which were driven through London to the Tower, to the general gratification of the Protector's subjects.