In 1386, the Treaty of Windsor was signed between England and Portugal, as a display of the countries mutual friendship, and can now be found on display in the National Museum of Portugal. Fast forward just under 600 years to WWII, the Prime Minister of Portugal, Antonio de Oliveira Salazar, was faced with a crucial decision: Should he break the treaty signed so long ago, and ally with the Axis Powers?
The decision was surprisingly easy for Salazar - he would honor the Anglo-Portuguese Alliance by refusing to sign the Anti-Comintern Pact, which was the document allying Germany, Japan, and Italy.
On June 29, 1940, Spain and Portugal signed an additional protocol known as a Treaty of Friendship and Non-Aggression. This document declared to the world that these two nations were going to stay out of the war, which didn't please Adolf Hitler.
In fact, a plan known as Operation Isabella was going to be executed in 1942, after the collapse of the Soviet Union. The plan involved invading Portugal and Spain and using them as air bases to strike Britain. Thankfully for Portugal, the plan was never executed.