NATO Article 5 has only been invoked once. This was in support of the US after 9/11.

[suffragette]

How deeply ironic that the United States has been the sole country to have invoked Article 5 and had the member nations of NATO come stand shoulder to shoulder in support only to have now a President who verbally rejects Article 5 and uses as example his unwillingness to support Montenegro, a NATO member that Russia has been targeting.

https://www.cnn.com/2018/07/18/politics/montenegro-nato-article-5/index.html

Article 5 has only been invoked once, in support of the United States after the 9/11 attacks. This led to NATO's largest-ever military operation, in Afghanistan, where hundreds of thousands of Europeans and Canadians stood shoulder-to-shoulder with US troops and more than 1,000 paid the ultimate price," the NATO official told CNN.
~~~
Trump, in an interview on Fox News, appeared to cast doubt on his willingness to defend the country, calling the people of Montenegro "strong" and "aggressive," suggesting that its aggressiveness could draw the US into World War III due to the NATO's mutual security clause enshrined in Article 5 of the NATO treaty. It deems an attack on one member of NATO an attack on all countries in the alliance.
Article 5 -- which is defensive -- 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.
"Our collective defense clause, Article 5, is unconditional and iron-clad. It means that an attack on one is an attack on all. President Trump has made clear that the US is fully committed to NATO and our Alliance is stronger than ever," a NATO official told CNN.

[2naSalit]

1. Indeed...nt