The world's largest military alliance, the North Atlantic Treaty Organization, or NATO, is turning 70 on April 4 this year. 

Currently, the organization has 29 members. In February this year, NATO states signed an agreement with Macedonia, now officially known as the Republic of North Macedonia, clearing the way for the Balkan nation to become the military alliance's 30th member.

Founded in 1949 by Western nations to counter the expanding influence of the then Soviet Union, NATO states that it seeks to guarantee the freedom and security of its members through political and military means.

NATO is committed to the principle that an attack against one or several of its members is an attack against all. 

This is the principle of collective defense, which is enshrined in Article 5 of the Washington Treaty - the accord on which the alliance was founded.

After the fall of the Berlin Wall in 1989 and the ensuing collapse of the Soviet Union, NATO lost its main adversary. 

From the late 1990s, the military alliance expanded eastward into the former Soviet Union's sphere of influence, incorporating a dozen Eastern European and Balkan countries as members. 

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NATO at 70: Will it die a natural death?

Here's a look at how NATO's membership has developed towards the east, and some highlights from its 70 years.