What is Article 5 of the NATO charter?

It is my understanding that it is an agreement between all members of NATO, that if one is attacked, they will see it as an attack on all.

For the first time since NATO was created, an American President has failed to to support Article 5. This has sent shock waves through our European allies.

We may as well have sent Vladimir Putin to represent us in the G-7 talks. He could have done no worse.