The principle of collective defence is at the heart of the North Atlantic Treaty – NATO's founding treaty, binding together its 30 member countries.
In 1949, the primary aim for the creation of NATO was to create a pact of mutual assistance in response to the threat that the Soviet Union would seek to extend its control of Eastern Europe to other parts of the continent.
Under Article 5 "The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."
If triggered, Article 5 allows such actions as it deems necessary, including the use of armed force to restore and maintain the security of the North Atlantic area.
It has only been activated once, in response to the 9/11 terror attacks on New York and Washington, on 11 September 2001.
Dr Jamie Shea, who was a member of the International Staff of NATO for 38 years, told Forces News: "What an armed attack actually is these days is broader than it was in 1949.
"NATO invoked Article 5... for the first and, so far, only time in its history, on the 12 September, immediately after the 9/11 terrorist attacks against the United States.
"NATO, subsequently, has declared that a cyber attack, if it had a certain level of severity and really did a lot of damage, could also be considered as a case of collective defence and trigger Article 5.
"So the spectrum is quite broad, it depends on what you call an armed attack. It can be with electrons, through cyber space as much as through tank shells and missiles.
"But at the end of the day, the determination of if it qualifies or doesn't qualify is going to be made by the 30 countries collectively around a table of the North Atlantic Council in Brussels."