|King John I of Portugal weds Philippa of Lancaster, 1386|
(Chronique de France et d'Angleterre, Jean Wavrin, 15th c.)
It didn't hurt Edward to have a friend on the continent. It gave him a potential place to land ships if he needed to march through (or against) Spain for any reason. The alliance helped Portugal as well. During political troubles in the 1380s, John the Good (called João in Portugese) defeated his rivals; England recognized him as the rightful king of Portugal right away. Spain would not recognize his right to rule until decades later. England reinforced the 1373 treaty in 1386 with the Treaty of Windsor, as a result of which King John I of Portugal married Philippa of Lancaster, daughter of John of Gaunt, Edward III's 4th son.
Does the treaty have any real significance in the modern world, considering the United Nations, NATO, etc.? Well, it is said that during World War II, Portugal refused to join the Axis powers in order to stay loyal to a 550-year-old document!
In the first paragraph, I said the treaty had been in place for "almost" 600 years, but wouldn't the date of 1373 mean it was in place for 640 years? Or, given that the treaty wasn't ratified until 1386, couldn't I have said "over" or "more than" 600 years? Well, I must be honest: the treaty wasn't always in effect during that time. From 1580-1640, due to marriages between the royal families of Spain and Portugal, Portugal was obligated to drop the treaty with England, Spain's enemy at the time.