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. Ukraine and Georgia, the two countries Russia has invaded in the past decade, are not NATO members.

Source: CNN July 06, 2017 15:18 UTC