The U.S. Treasury has stepped in to help Japan support its currency, a rare move that signals deep stress in the global economy.

• The U.S. is buying Japanese yen alongside Japan to stop the currency from crashing, something governments almost never do.
• Japan's currency is falling because of weak growth, an aging population, and high oil costs that make its imports more expensive.
• The worry is that a desperate Japan could sell off its big pile of U.S. government debt, which would push American interest rates sharply higher.
• Oil prices dropped after Trump held off on attacking Iran, easing some pressure, and there is talk of new negotiations.
• Korea's stock market has also crashed, with small investors calling it a casino and swearing off stocks — a warning sign that the strain is spreading across Asia.

Outlook: More joint yen buying is expected, and mounting financial pain may be what finally pushes Trump to avoid a costly war with Iran.