Articles four and five of the NATO founding treaty of April 4, 1949 are core elements of the defense alliance. Above all, Article Five is considered the heart - it regulates the so-called alliance case.
Article Four states: "The parties shall consult each other if, in the opinion of either party, there is a threat to the integrity of the territory, to the political independence or to the security of either party." This does not have to result in concrete reactions.
Article Five provides that the Allies regard an armed attack against one or more of them as an attack against all of them. You agree to provide assistance. Specifically, it states that it is about the measures they consider necessary to restore and maintain the security of the North Atlantic area - including armed force.
Article Five has only been activated once in NATO's face, after the attacks of September 11, 2001. This led to Germany and other NATO countries taking part in the war against the Taliban and the Al-Qaeda terrorist organization in Afghanistan . However, the wording of the article does not state that NATO countries have to send their own troops to provide support, for example. It only obliges to take immediate measures that the respective NATO state deems necessary.