NATO’s Article 5 is the principle of collective defense, stating that an attack against one NATO member is considered an attack against all members. However, the invocation of Article 5 in the event of an attack by one member against another is unprecedented and would present a complex legal and political situation.
If the USA, a founding member and key leader of NATO, were to invade Canada, another NATO member, it would challenge the essence of Article 5. Traditionally, Article 5 is designed to deter external threats, not conflicts between members. NATO would need to scrutinize whether Article 5 applies in this intra-alliance conflict.
In practice, the other NATO members would likely convene urgently to discuss and address the breach of alliance principles. The focus would initially be on diplomatic solutions to restore peace, leveraging political pressure and negotiations to halt hostilities. This scenario would strain relations within NATO severely and test its cohesion and relevance. Additionally, the UN and other international bodies could become involved to mediate and resolve the conflict to preserve international stability and order.
