The principle that an attack on one member of NATO is an attack on all of its members is the Article 5 maxim, that prompted Estonia's drive for membership. NATO's collective umbrella was an absolute necessity for all the central and east European countries in shaking off any lingering bits of Soviet legacy and getting reassurance of protection against Russian aggression, perceived or real.
Today, 70 years after the 29 member defence alliance was established (12 original members signed the North Atlantic Charter in 1949), the relevancy of Article 5 has not diminished. During the Cold War, of genuine concern was the hostile stance of the Soviet Union. It's the Russian invasion of Georgia and Ukraine, both non-NATO states that don't qualify for Article 5's protection, that has still kept Article 5 entirely relevant.
Of critical importance is Article 5's universally accepted intent – to deter potential aggressors from attacking any NATO member. It has been invoked only once – after the Sept 11 terror attacks on the US. This was the first time that NATO military assets were deployed in support of an Article 5 operation. One could interpret this rare incidence of a violent assault against a NATO member as Article 5's undeniable success in deterrence.
That was in 2001. The signals from Washington, regarding the efficacy of NATO itself, during the last few years have been, at the very least, confusing. The US president has been a harsh critic of NATO overall, even calling it 'obsolete'. His reassurances of unwavering US commitment to a strong NATO and of NATO deployment in the Baltic states was given to the three Baltic presidents just one year ago in April 2018 as had other presidents in 1998 and 2013. He also stressed that he wanted better relations with Russia while commenting that only 'stupid people' would think otherwise, while emphasizing that no one was tougher on Russia than the US.