City of Cadiz overrun with drunken Brits ...

 ... No, our provincial capital hasn't turned into Magaluf. We're talking about 1625, when Spain and England were at war over something or other. (If you really want to know see Anglo-Spanish War (1625–1630.)

George Villiers, Duke of Buckingham and Lord High Admiral, wanted to destroy Spain's economy by raiding ships laden with gold and silver from the Americas and taking over the port towns such as Cadiz. Around a hundred ships and 15,000 men were readied for the Cadiz Expedition, led by Sir Edward Cecil - a proven soldier but with no experience of naval warfare.

They left Plymouth on 6 October 1625 but bad weather in the Bay of Biscay caused delays and damaged many ships. By the time they reached Cadiz they had nearly run out of supplies. 

Cecil was surprised by how well the city was fortified (after the English pirate Francis Drake "singed the King of Spain's beard" there in 1587, and Anglo-Dutch forces burned the city to the ground in 1596, the gaditanos weren't taking any chances as far as the English were concerned).  Nonetheless Cecil was able to capture the fort that guarded the harbour and send his men ashore - an English merchant assured him that the city was defenseless, as the Spanish forces had departed by ship for Puerto Real. 

As the exhausted men had nothing to eat or drink, Cecil allowed them to go and explore the gastronomic delights of the city.  The men soon discovered that every house contained vats of sherry wine, the area's tipple of choice, and as we all know, drinking on an empty stomach isn't a great idea. 

Realising his mistake, Cecil ordered them to return to their ships.  When the Spanish army returned soon afterwards they found the English soldiers, though armed, were too drunk to resist. Not a single shot was fired as the Spanish put over a thousand men to the sword.

Cecil's run of bad luck (or judgement) was not over.  Having failed to take Cadiz he took his ships out to intercept a fleet of Spanish galleons returning from the Americas.  However they had been warned of the potential threat, and took another route home.

Edward Cecil, 1st Viscount Wimbledon

The failure of the Cadiz Expedition had further repercussions back in England.  Charles I turned a blind eye but the House of Commons began the process of impeachment against Buckingham, a favourite of the King; Charles chose to dissolve Parliament rather than risk his friend's prosecution.  Cecil, in the meantime, was given a peerage and became Viscount Wimbledon.  And sherry became the tipple of choice not only in Cadiz but across the stately homes of England.