Here’s something more miraculous than Congress spending less money—the United Nations doing it. Yet that’s what happened at the end of 2017 as the 193-nation General Assembly agreed to a 5% spending cut in its new biennial budget after American prodding.

The General Assembly agreed by consensus to shrink the U.N. budget by $286 million, to $5.4 billion, down 5% from the prior budget. The U.N. will save about $50 million by trimming hiring and overhead costs, and another $18 million from cutting the U.N. Department of Management,...