Fri May 26, 2017 AT 1:20 PM EDT
President Trump had been expected to promise that America would defend its NATO allies if they ever came under attack. That principle of collective defense is, in theory, cemented by Article 5 of the alliance’s charter, NATO’s core tenet. It means that “an attack against one ally is considered as an attack against all.” No other president since NATO was founded in 1949 has questioned that principle — until Trump, reports NBC.