In 1541,
Francis I of France violated the
Truce of Nice, allied with the
Ottoman Empire,
Denmark, and
Sweden, and declared war on
Spain. To be ready for war, Francis I made preparations to fight the navies of both
Spain and
England. As a part of the preparations, Francis I personally went to the port of
Le Havre to oversee the preparations for war. In addition, French Vice Admiral De Bury ordered the assembly of a second French fleet to attack Spain enlisting the ports of
Bayonne and
Bordeaux.
Emperor Charles I of Spain fearing action in these waters ordered Álvaro de Bazán to proceed to the Bay of Biscay and assemble a fleet to try to prevent a French attack. In the Bay of Biscay Álvaro de Bazán created a fleet of vessels from
Guipuzcoa and
Vizcaya and stationed his fleet outside the port of
Laredo, within reach of both the French and
Cantabrian coast. During this time, Álvaro de Bazán was ordered to transport infantry units to
Flanders so used fifteen of his ships to transport troops to the city of
Bruges where he delivered the infantry units at the end of June. While Álvaro de Bazán was still in Flanders, the French finished their naval preparations and sent thirty specially armed ships from the port of Bayonne to attack the Spanish coast. Although the French fleet had been formed by Vice Admiral De Bury, it was sent to attack the Spanish under the command of France's greatest seaman,
Jean de Clamorgan. On the coast of
Galicia the French fleet captured two Spanish merchant ships, an error which caused Spanish Governor, Sancho de Leyva to send a warning to Álvaro de Bazán. At that time, Álvaro de Bazán was busy attempting to complete fitting out. Upon receiving the warning from Governor Leyva, he asked for and received reinforcements which helped him to complete the work on his ships. On 10 July, the French fleet passed through the waters of
Laredo, looted numerous villages including Lage, Corcubión, and held the city of
Muros at ransom. Given the urgency of the situation, Álvaro de Bazán sailed to Muros as fast as possible in an attempt to catch the French off guard. . ==Battle==