Intervention is unlikely to do much harm because the yen is, according to most estimates, extremely cheap. Japan has large foreign currency reserves — accumulated decades ago, during interventions when it was trying to hold the yen down, rather than prop it up — and converting some of those reserves back into yen at today’s advantageous exchange rates will realise a sizeable profit. As long as Japan does not try to fight the markets with massive, open-ended intervention, purchases now will most likely work out as a good trade, and not something the country has cause to regret.