Complex sentence: The border crossing into the United States at Chief Mountain is only open from mid-May through mid-September, entering into Glacier National Park, crossing the Divide back to the western side on the spectacular Going-to-the-Sun Road.
Split into simpler sentences: The border crossing into the United States at Chief Mountain is only open from mid-May through mid-September. From there, the route enters Glacier National Park, crossing the Divide back to the western side on the Going-to-the-Sun Road.