Portugal

Portugal concluded a military alliance with England, which is considered the oldest alliance still in force in the world.

An excerpt from the article 37 facts about Portugal

This alliance was ratified in the Treaty of Windsor in 1386. Since the signing of the treaty, the Kingdom of Portugal and the Kingdom of England have never been at war with each other or engaged in wars on opposing sides as independent states.

Today, Portugal and the UK are joint members of NATO.