There were plenty of examples of countries devaluing their currency before World War I. No one was ever really on the gold standard, that was just what the governments were willing to exchange it for at times. The government could always alter the exchange rate or simply stop exchanging it for gold even if they were "on the gold standard." (Look at the Confederate dollar for an example). Also, the only reason World War I is the first World War is because we called it that. There were plenty of wars which involved multiple continents before that. For example, the Napoleonic Wars involved every continent but Antarctica.