How the United States might answer an 18-inch British ship

How would the US Navy respond if Britain fielded an 18-inch-gunned battleship before the First World War?

It would probably field more 16-inch guns first and wait until a technically viable American 18-inch weapon was ready.

A British 18-inch HMS Agincourt would probably prompt the United States to increase the number of proven 16-inch guns, much as it answered the 15-inch Queen Elizabeths with more 14-inch weapons. American designers would want to reach 18 inches but would wait until their technology could support it. The British 1.5-inch progression and American 2-inch progression would eventually converge at 18 inches. They would not necessarily reach that calibre in the same design generation.