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Nato, or the North Atlantic Treaty Organisation, was founded at the start of the Cold War to deter the Soviet Union from expanding and to prevent the rise of nationalist militarism in Europe.
Twelve founding nations launched Nato in 1949 and its ranks have now swelled to 30 countries (28 from Europe and two from North America). Three former Soviet republics — Latvia, Lithuania and Estonia — are members.
Is Ukraine in Nato?
Ukraine, itself a former Soviet republic, is not a member of the alliance.
In 2008, Nato promised that Ukraine, as well as Georgia, could join Nato. The alliance, however, did not specify a time frame.
The pledge infuriated Russian President Vladimir Putin, who claimed it betrayed a Nato promise not to expand eastward. The purported Nato assurance is disputed.
Ukraine is instead a partner of Nato.
As an alliance partner, Ukraine contributes to promoting democratic norms and supporting Nato-led missions. The organisation's programmes also help partner countries build their own defensive and security forces.
Ukrainian President Volodymyr Zelenskyy in 2020 approved his country's new National Security Strategy “with the aim of membership in Nato”, the alliance said.
And while Ukraine established relations with Nato in 1992 and has contributed to missions in Afghanistan and Iraq, it does not benefit from the organisation's collective defence commitment.
Under Article Five of the Nato agreement, an attack on one member of the alliance is considered an attack on all members.
This principle binds the 30 member countries to the concept of mutual protection, which is a cornerstone of the alliance. The only time it has been invoked was by the US in 2001, following the September 11 attacks.
US President Joe Biden reaffirmed America's commitment to Nato after Russia launched its invasion into Ukraine. His predecessor, Donald Trump, repeatedly bashed the alliance, accusing other members of not contributing enough towards defence.
“As I made crystal clear, the United States will defend every inch of Nato territory with the full force of American power,” Mr Biden previously said.
The US and its allies have supported Ukraine by supplying it with military equipment and have responded to the Russian invasion by issuing scores of sanctions.
However, because Ukraine is not a member, Nato has no remit to directly intervene in the conflict with Russia.