Trump was speaking at the annual Rolling Thunder motorcycle rally, which honors prisoners of war and service members missing in action.
Trump was warmly received by the crowd despite having once said he prefers "people who weren't captured" when criticizing Arizona Senator John McCain, a war prisoner during Vietnam.
Trump told the crowd that if elected he plans to "knock the hell out of" the Islamic group by building a bigger, better military.
He also reiterated his promise to build a wall to keep out people from entering the country illegally. "Who's going to pay for the wall?" he bellowed. The crowd yelled back: "Mexico." ''Not even a doubt!" he said.