The Methuen treaty was negotiated by John Methuen (c.1650-1706) who served as a Member of Parliament; Lord Chancellor of Ireland; Privy Councilor; Envoy and then Ambassador Extraordinary to Portugal where he negotiated the "Methuen" Treaty of 1703 which cemented allegiances in the War of Spanish Succession.
The early years of the War of Spanish Succession, in Flanders, had been rather fruitless. The Tory party in England was concerned about the cost of the war and felt that naval warfare was a much cheaper option with greater potential for success. Portugal offered the advantage of a deep water ports near the Mediterranean which could be used to counter the French Naval base at Toulon.
Until 1703, the Grand Alliance had never established any formal war aims. The Methuen Treaty changed this as it confirmed that the alliance would try to secure the entire Spanish Empire for the Austrian claimant to the throne, the Archduke Charles, later Charles VI of Austria.
The treaty also established the numbers of troops the various countries would provide to fight the campaign in Spain. The Portuguese army was very poorly organized and equipped and so they had to depend on funding from England. The Portuguese also insisted that Archduke Charles would come to Portugal to lead the forces in order to ensure full allied commitment to the war in Spain.
In addition, the treaty helped to establish trading relations between England and Portugal. The terms of the treaty allowed English woolen cloth to be admitted into Portugal free of duty. In return, Portuguese wines imported into England would be subject to a third less duty than wines imported from France. This was particularly important in helping the development of the Port industry. As England was at war with France it became increasingly difficult to acquire wine and so port started to become a popular replacement.