WASHINGTON — The United States is deepening its involvement in the war against the Islamic State after an unprecedented American airlift of Arab and Kurdish fighters to the front lines in northern Syria, supported by the first use of U.S. attack helicopters and artillery in the country. The U.S. forces didn’t engage in ground combat, but the offensive suggests the Trump administration is taking an increasingly aggressive approach as it plans an upcoming assault on the extremists’ self-declared capital of Raqqa. In addition to using helicopters to ferry rebels into combat near the Tabqa Dam on the Euphrates River, the U.S.