The United States is convinced that it has the right to repel aggression. Should a country demonstrate a trend not to obey any of the NATO members, the USA is there to interfere into the conflict immediately, said Douglas Lute, Washington's permanent representative to the North Atlantic Alliance.
Against the background of the Ukrainian crisis, Russia apparently poses the largest threat to the United States. Therefore, the statement about full readiness to counterattack was most likely made for Moscow.
In order to let NATO launch responsive operations to aggression, one needs to obtain unanimous approval from 28 countries first. However, it appears now that the USA does not need such a mandate.