Should NATO invoke Article 5? This has become one of many burning questions in the aftermath of the horrendous terrorist attacks in Paris, which left 129 people dead and more than 300 wounded. The answer may well be that NATO should invoke this important collective defence provision. But it is important to understand what invoking Article 5 would — and would not — mean.

Article 5 is the foundation of the Washington Treaty, signed by the then-twelve members in 1949. It states that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” In the more than 66 years since its adoption, this provision has been invoked only once — in the immediate aftermath of the September 11 attacks, when NATO countries agreed that if it was determined that these were directed from outside the United States, the 9/11 attacks would be covered by Article 5. Read more