Defense Secretary James N. Mattis said Monday Pakistan must take on a substantive role in peace talks with the Taliban if the war in neighboring Afghanistan is to be ended. Speaking to reporters at the Pentagon Monday, Mr. Mattis demanded Islamabad provide explicit support for “all those who are trying to maintain peace” in Afghanistan and elsewhere in the region. “We are looking for every responsible nation to support peace in the subcontinent and across this war in Afghanistan. … It is time for everyone to get on board,” Mr. Mattis said ahead of bilateral talks with Indian Defense Minister...