No sooner had Bank of Japan Gov. Haruhiko Kuroda finished insisting that ultraeasy money was here to stay than authorities swept in to mop up the damage.

If there was ever any doubt that the primary cause of the yen's collapse this year was the BOJ's prolonged easing in the face of interest rate hikes pretty much everywhere else, it was wiped away Thursday.

The Ministry of Finance, which directs Japan's currency policy, intervened to boost the beleaguered yen for the first time in decades. The purchases strengthened the yen through 141 per dollar; the currency had earlier weakened past 145, the cheapest it had been since the Asian financial crisis of the late 1990s.