Typically at that time late in the cycle the leading empire that won the last economic and geopolitical war is less powerful relative to rival powers that prospered during the prosperous period, and with the bad economic conditions and the disagreements between powers there is typically some kind of war.
Out of these debt, economic, domestic, and world-order breakdowns that take the forms of revolutions and wars come new winners and losers. Then the winners get together to create the new domestic and world orders. United States is now the most powerful empire by not much, it is in relative decline, Chinese power is rapidly rising, and no other powers come close.