Story highlights Article 5 aims to deter potential adversaries from attacking NATO members

It is the principle that an attack on one member pf NATO is an attack on all members

(CNN) President Donald Trump, during a speech in Poland on Thursday, articulated his most strident commitment to NATO's Article 5 principle of common defense, a key tenet of the alliance that Trump hasn't always been so keen to support.

Article 5 is the principle that an attack on one member of NATO is an attack on all members. It's been a cornerstone for the 29-member alliance since it was founded in 1949 as a counterweight to the Soviet Union.

Article 5 has only been invoked once: After the September 11, 2001, terror attacks on the US.

Article 5 aims to deter potential adversaries from attacking NATO members. During the Cold War, the main concern was the Soviet Union, but in recent years, Russia's aggressive actions in Eastern Europe have been the focus of attention. Ukraine and Georgia, the two countries Russia has invaded in the past decade, are not NATO members.

When Trump came into office, however, Eastern European countries like Poland were nervous about whether he would actually uphold the principle because of his campaign rhetoric.

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