The strategy that Germany had been using to pay war reparations was the mass printing of bank notes to buy foreign currency, which was then used to pay reparations, but this strategy greatly exacerbated the inflation of the paper mark. Since the mark was, by fall of 1922, practically worthless, it was impossible for Germany to buy foreign exchange or gold using paper marks. After Germany failed to pay France an installment of reparations on time in late 1922, French and Belgian troops occupied the Ruhr valley, Germany's main industrial region, in January 1923. Reparations were to be paid in goods, such as coal, and the occupation was supposed to ensure reparations payments.