For decades, the 29 countries making up NATO have been reassured by the treaty‘s ironclad guarantee of mutual defence in Article Five of its founding charter: „an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.“

But in the era of U.S. President Donald Trump, governments now have doubts about the United States‘ commitment to Article Five. The mutual defence clause has only ever been invoked once — by Canada on behalf of the U.S. in the immediate aftermath of the September 11, 2001 terrorist attacks.