The Alliance was founded with the signature of the North Atlantic Treaty on April 4, 1949. Portugal is among the founding nations of the organisation, together with Belgium, Canada, Denmark, France, Netherlands, Iceland, Italy, Luxembourg, Norway and the United Kingdom. Today the Alliance is composed of 29 members. The North Atlantic Treaty can be read here.
One of NATO's key documents is the Strategic Concept. In it the Alliance presents its strategic vision for the future, so that the organisation is prepared to face the current challenges by making it more flexible, adaptable and efficient. The latest Strategic Concept was adopted at the 2010 NATO Summit in Lisbon and can be found here.
Also noteworthy are the official Communiqués of the Summits of Heads of State and Government which establish the most immediate priorities of the Alliance and guide the work of the NATO Structures for the following years. The last Summit took place in Warsaw in July 2016, and the final communiqué can be found here.
Other NATO official texts can be found on the organisation's website.