Treaty: Anglo-Portuguese Alliance of 1373

One of the documents that still survives from Medieval times is a copy of this treaty, agreed to between King Edward III of England and King Ferdinand I and Queen Eleanor of Portugal.  The fact that the Queen Consort of Portugal was included in the treaty is unusual enough.  Queen Philippa of England had died in 1369, meaning that Edward was a widow.  So, we don't know if she would have been included in the wording as a matter of courtesy.

England had a serious rival in France and they were in the midst of the Hundred Years War when this treaty was sign.  England had need for an international partner who had ports for trade and defense.  And Portugal was ever fearful of the Castilians, the Aragonese, and God forbid, the Moors of Africa.  The treaty did not settle any disputes between the Kingdoms, but was a promise of mutual aid and defense.  An attack on Portugal would be grounds for an attack by England and vice versa.  The Treaty survived the centuries and was reiterated several times, only becoming void between 1580-1640, when the Crown of Portugal was also held by the Kingdom of Spain, another of England's sworn enemies. 

This treaty came in handy for Portugal during the Napoleonic era, when England sent the Duke of Wellington and thousands of British troops, plus the Royal Navy, to defend Portugal against French onslaughts through Spain, then a Bonapartist puppet kingdom under Napoleon's brother, Joseph.  And, Portugal would call on England's aid again in maintaining neutrality during World War II.  Britain protected Portuguese neutrality in return for use of landing rights for aircraft combating Germany U-boats.