The yen has surrendered nearly half its gains from the historic joint US-Japan intervention, weakening 0.7% to ¥158.91 on Monday after bouncing from a four-decade low of ¥164 to near ¥155 in late July. Traders maintain bearish bets though positions have shrunk since intervention, per CFTC data.

Van Luu of Russell Investments said the effect is fading and "going to take more" for a sustained upswing. Investors argue lasting support requires Bank of Japan rate hikes and international coordination. Guy Miller of Zurich criticized the ECB's exclusion, noting coordination signals a "unified voice," unlike the 2011 G7 effort.

Focus shifts to whether the BoJ will raise rates. Goldman Sachs analysts noted July meeting opinions show risks "clearly skewed" to earlier hikes, with one member suggesting a faster pace than expected. Traders price a 50% chance of a quarter-point hike in September; Citi forecasts a regime shift to 2% by end-2026.

A slide back through ¥160 would worsen inflation and US concerns over dollar strength and potential Japanese Treasury sales. Conditions resemble August 2024 when yen appreciation sparked volatility via carry trade unwinds. But Ayako Fujita of JPMorgan sees rapid unwind unlikely given wide rate differentials.