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.
Example such as? Because the big players like Europe and USA all did during WW1, then got back to the gold standard just after the war for a few years until John Maynard Keynes convinced everyone that it was a bad idea, and we're still debating whether that's the case to this day.
I don't recall the specifics but only the USA was in a weird fiat-gold hybrid until the 70s as before that they guaranteed they would buy gold at a specific price in $
