The US is quietly stepping in to support Japan's falling currency, a sign of how fragile America's own finances have become.

• Japan's yen has fallen to its weakest level in 40 years, pushed down by heavy government debt and aggressive spending.
• The US is helping Japan partly to help itself: Japan holds over $1.1 trillion in US government bonds and could dump them to fund its own rescue.
• If Japan sells those bonds, US borrowing costs jump — 30-year yields already hit their highest since 2007.
• US debt has topped $39 trillion and now costs over $1 trillion a year in interest, more than the entire defense budget.
• Even Steve Bannon is now attacking the Trump team over runaway deficits, with a $2 trillion shortfall expected this year.

Outlook: If the yen keeps sliding, the US risks losing money on the bailout, but pulling back risks a bond-market shock — a bind with no easy exit.