Answer:
Unpaid WWI debts.
Explanation:
During WWI, USA loaned money to Allies such as United Kingdom, France, Belgium etc, so they can provide themselves with weaponry and other war material. After the war was finished, Allies were broke and they were not in the situation to pay back loans and they relied on Germany who had the obligation to pay the reparation costs for all the damage in WWI. But Germany was in a bad situation too, so they lent money from the USA so they can meet its obligations. So all ll of them were sucked into this vicious circle. USA also collapsed thanks to loans of these two sides. Germany was pressed to come up with the money, but its industry was shut down, they tried to solve this problem by printing a lot of money, but that only caused hyperinflation. So, as Hoover said: We are now faced with the problem, not of saving Germany or Britain, but of saving ourselves.
