THE UNITED NATIONS has known many crises since its founding in 1945, from the mysterious death in Congo of its secretary-general, Dag Hammarskjöld, in 1961, to Nikita Khrushchev’s alleged shoe-banging in the cold war, the massacres of civilians under its protection in the 1990s, and America’s invasion of Iraq in 2003. As leaders prepare to gather in New York next week for the UN’s 80th birthday, veterans say none of those disasters feels as calamitous as this one. With President Donald Trump’s return to the White House, there are many traumatic scenarios for the un, but three stand out: going rogue, decay and turning Trumpian.