I know a little bit about this so maybe I can help. The Anglo-Portuguese alliance is the oldest continuously maintained alliance in the world, dating back to the fourteenth century. In 1373 a treaty of perpetual friendship was signed between Edward III of England and Fernando and Leonor of Portugal.

Basically what happened was: In the mid to late fourteenth century the Kings of England and France were both competing to win an alliance with Castile which was the largest of the Iberian kingdoms (now part of Spain). The Castilians were a formidable naval power and would be a distinct advantage to either side in the Hundred Years War.

In 1366 the Castilian Civil War broke out between supporters of the King, Pedro the Cruel (backed by the English) and supporters of his illegitimate half-brother Enrique Trastámara (backed by the French). Pedro was assassinated by Enrique in March 1369 and Enrique was proclaimed King of Castile.

Pedro’s three daughters: Beatriz, Constanza and Isabel had fled to Portugal earlier in the Civil War. The eldest became a nun and died in 1369 but the youngest two were sent to England where they married the princes John of Gaunt and Edmund of Langley.

In 1373, with an alliance with Castile out of the question Edward III of England entered into an alliance with the Portuguese with the aim both of countering the Franco-Castilian alliance and of using Portuguese assistance to claim the Castilian throne for England.

In 1386 John of Gaunt launched his campaign for the Castilian crown which he aimed to claim through his marriage to Constanza, Pedro’s eldest surviving daughter and heir. Gaunt’s attempt on the crown was a complete failure and he returned home to England in 1387, leaving behind his eldest daughter from his first marriage, Philippa who reinforced the treaty by marrying Joao I in 1386. Philippa was very popular in Portugal and Joao’s reign ushered in what was known as the Illustrious Generation, producing six surviving children including Henry the Navigator. Philippa played a large role in the continuance of the alliance during her lifetime serving both English and Portuguese interests particularly commercially, by acting as a negotiator/liaising between the two countries.

The treaty was useful for both sides during the following centuries with the English coming to the aid of the Portuguese during the Seven Years’ War and the Napoleonic Wars and the Portuguese providing assistance to the Allies in both World Wars.

As far as frayings go: the alliance was void from 1580 to 1640 when Portugal and Spain were ruled by the same monarch, but it was reinstated after Portugal regained its independence. The relationship was strained in 1890 with the British Ultimatum which was a dispute over territories claimed in Africa.

There's an excellent book which I recommend you check out by P. E. Russell called 'The English Intervention in Spain and Portugal in the Time of Edward III and Richard II' which provides a narrative analysis of the beginnings of the treaty. Although the book is a bit old now (published in 1955) and quite long, its an enjoyable read and probably the best one you'll find on the topic.

If I may, I'd like to discuss the Portuguese situation in WW1. They allied themselves with Britain and ultimately lost a substantial amount of men on the western front in 1916-1918.

Basically, in 1914 Portugal owned a number of (what they considered to be) important colonies abroad, in Africa and Asia (such as Angola or Macao). They were in competition with the Germans in these areas and thus felt any chance to solidify their position was worth taking. As imperial opponents to the Germans, they made a natural ally for the British/French.

Ultimately, there is a a massive upheaval of government in Portugal in late 1917 which leaves the entire Portuguese Corps up in the air so to speak (I am hazy on the political side of things so anyone with more knowledge please help). They generally get criticized heavily in our anglophone historiography for their defeat by the Germans in the [Spring of 1918](http://en.wikipedia.org/wiki/Battle_of_the_Lys_(1918), but conveniently forgotten is that this was a poorly-led, arguably demoralized force of 20,000 men (at a generous estimate) facing a determined attack by 8 German Divisions (approx. 100,000 men) with massively superior artillery. Of course they could not hold them off. The British forces further to the south had capitulated in a similar fashion. A massively depressing monument to this episode is the Portuguese Cemetery in Neuve Chapelle, which is full of decaying headstones, some of which have completely fallen over (apparently the current government can't afford the upkeep). It serves as a visceral reminder of the way in which these men were abandoned by the government that sent them into such a charnel-house in the first place.

So I guess a TL;DR to my post: early 20th Century, Portugal still has some imperial possessions/colonies that they want to defend from the Germans. The British are their natural choice to support in WW1 and their soldiers suffer accordingly.

EDIT: I have personal pictures from the Portuguese Cemetery is anyone is interested.

Somebody's been playing EU IV!

To extend this question, did Portuguese collonies, specially Brazil, inherited the deal? As a Brazillian, we learn that our relationship with England was very unbalanced during the 19th century, but don't learn on how we benefitted (if we did) as Portugal.

Did the British send any assistance after the Lisbon earthquake, or was this purely military?