The arrival of 2,500 Marines and another 2,500 sailors is keeping the number of American troops in the Mideast region at over 50,000 — roughly 10,000 more than usual — as President Trump decides on his next step in his month-old war in Iran. While it is still unclear just what the Marines, from the 31st Marine Expeditionary Unit, will be charged with, U.S. officials say the president is weighing whether to try a larger attack, like venturing to seize an island or other ground as part of Mr. Trump’s effort to open the Strait of Hormuz. The narrow waterway,...