After the European settlement, strange ideas about "manifest destiny" were used as an excuse to push Native Americans from their land. Later, in the antebellum South, some preachers routinely endorsed the thesis that slavery, American-style, was endorsed by God. Today's "exceptionalists" are really no different, a theologian's equivalent of the child in the backseat telling his sister that "Mom likes me better than you!"
God doesn't play favorites among nations. Any "exceptionalism" in the United States is because of the character of her people and a structure of government which doesn't pick sides in theological debates either. When we start thinking that God is patting us on the head more regularly, we'll be "exceptional" only for our vanity and pride.