According to what agreement would the united states have to declare war against any nation that attacked great britain?

Answer: The North Atlantic Treaty

Explanation: The North Atlantic Treaty Organization (NATO), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 North American and European countries which implements the North Atlantic Treaty that was signed on 4 April 1949. This organisation was the first peacetime military alliance the United States entered into outside of the Western Hemisphere.

The organisation's essential and enduring purpose is to ensure and safeguard the freedom and security of all its members by political and/or military means. Collective defence is the bedrock of the Alliance and this has given rise to a spirit of solidarity among its member nations.