President Trump claims trade wars are easy to win, but that boast looks worse than ever amid the financial carnage from his latest threat of tariffs on Chinese goods. His trade war has now become a currency war, which raises the potential economic harm to another level.

That was the ugly message Monday in financial markets, which sold off world-wide after China devalued the yuan following Mr. Trump’s threat of 10% tariffs on $300 billion in Chinese goods. Emerging-market currencies plunged, equity markets dropped 3% or so...