Real wars have guns, and trade wars are fought with weapons such as tariffs. Currency wars, on the other hand, are stealth battles — no country ever admits that it’s waging one. They surface when policy makers are accused of deliberately driving down exchange rates — or fixing them too low — to gain a competitive advantage. A weaker currency means a country’s exports can be sold more cheaply overseas, providing a jump-start to the economy at home. Things really heat up, though, when suspicious nations retaliate. After years of largely unspoken tensions, U.S. President Donald Trump and his tweets have brought hostilities into the open, raising concern about an unraveling of decades of global pledges to refrain from combat using currencies.