The Methuen Treaty, or Port Wine Treaty, as it became known, was signed between Portugal and England in 1703, during the early years of the Spanish War of Succession, to establish trading relationships between the two countries.
At the beginning of that war Portugal had been aligned with France, who had promised to provide naval protection. However, when the English navy sailed dangerously close to Lisbon as it travelled to and from Cadiz, the Portuguese decided France wasn't keeping its side of the deal and it would be better to change sides.
The Methuen treaties, one military and the other commercial, were the result of that decision and were named in honour of lead negotiator John Methuen, an MP who was also England's Ambassador to Portugal at the time.
The Port Wine Treaty stipulated that English textiles, mainly woollen cloth, was to be admitted into Portugal free of tax and in return, Portuguese wines imported into England would be subject to a third less tax than wines imported from France.
This was particularly important in helping the port industry to develop. As England was at war with France, it became increasingly difficult to acquire wine, and so people started buying port instead.
This had a knock-on effect on the Spanish sherry industry in Jerez, in Cadiz province. The War of Succession and later Napoleonic wars resulted in sales to England and the Netherlands dropping considerably as the hostilities increased, and there was also a change in European tastes due to port becoming more accessible thanks to the Methuen Treaty.
Many of the sherry producers found themselves left with unsold stock ageing in oak barrels in their bodegas, but as a result the taste of this wine became more concentrated and nutty.
The producers would only bottle enough sherry as they needed according to demand, and then top up the barrel with some of their younger wine. This practice, which is now known as the famous 'solera', introduced new flavours and aromas and the wines developed new characters as they aged and became increasingly popular.