Article 5 of the North Atlantic Treaty has been a key point of discussion among world leaders amid tensions between Ukraine and Russia.
Article 5 of the treaty is the principle of collective defense. It guarantees that the resources of the whole alliance can be used to protect any single member nation. This is crucial for many of the smaller countries who would be defenseless without its allies. Iceland, for example, has no standing army.
Since the US is the largest and most powerful North Atlantic Treaty Organization member, any state in the alliance is effectively under US protection.
According to the NATO website, this is what Article 5 lays out:
"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 and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defense recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.”
In reality, the first and only time Article 5 has been invoked was in the aftermath of the September 11, 2001 attacks on the US; as a result, NATO allies joined the invasion of Afghanistan.
However, NATO has taken action on other occasions too.
It put collective defense measures in place in 1991 when it deployed Patriot missiles during the Gulf War, in 2003 during the crisis in Iraq, and in 2012 in response to the situation in Syria, also with Patriot missiles.
All three were based on requests from Turkey.
Read more about NATO and Article 5 here.