THE United Nations General Assembly has served for decades as a platform where American presidents sought to present a coherent vision for global order, peace and international cooperation. At the rostrum, American leaders exhibited statesmanship that signaled stability to allies and resolve to adversaries. On Sept. 22, President Donald Trump shattered that tradition with a deranged, low-energy speech in which he threatened to annihilate a sovereign nation against which he launched an unprovoked war seven months ago.