BRUSSELS—NATO ambassadors gathered Thursday morning for an emergency assembly in response to Russia’s invasion of Ukraine with a pledge to defend alliance customers and support Ukraine, which is not a member.

Russia’s assault on Ukraine, which because 2008 has had an invitation to possibly be a part of the alliance, has breathed new lifestyle into the North Atlantic Treaty Business, the U.S.-led military services alliance produced in the ashes of Earth War II to sq. off with Moscow.