What is its aim?

The effect of this agreement is to bind America to defend Europe. Any aggressor who attacks any Nato ally - whether Estonia or Canada or Greece - knows that it will also have to go to war with the United States.

The aim is to keep Europe safe by deterring attack.

America's permanent commitment to protect Europe, embodied by the Nato alliance, amounts to the cornerstone of the continent's security. This pledge is made still more formidable by the fact that America possesses the second biggest arsenal of nuclear weapons in the world.