Complex sentence: The phenomenon understandably raises a lot of concerns about the size of the U.S. external debts as a high, unsustainable external debt will result in painful debt services and/or sharp depreciation of the dollar.
Split into simpler sentences: The phenomenon understandably raises a lot of concerns about the size of the U.S. external debts. As a high, unsustainable external debt will result in painful debt services and/or sharp depreciation of the dollar.