Despite official statements against rapid currency depreciation in Japan and Korea, policymakers likely view a weaker currency as a beneficial stimulus. With negative output gaps and competition from China, the goal is not to reverse the trend but to manage its pace to avoid market disorder and US Treasury scrutiny.

The US-Japan yen intervention wasn't about establishing an unbreakable price cap. Instead, its primary goal was psychological: to make speculators nervous about shorting the yen near the 160 JPY/USD level, thereby restoring the Ministry of Finance's perceived threat.

Japan's Ministry of Finance (MOF) tactically delayed its yen-buying intervention. Instead of acting when the yen first weakened, it waited for the broad US dollar sell-off following the FOMC meeting. This allowed them to amplify an existing trend, maximizing the intervention's effectiveness and market impact.

The US Treasury's intervention was not just about the Yen's exchange rate or trade balance. A primary motive was to prevent Japan from being forced to sell its vast US Treasury reserves to fund its own intervention, which could create significant pressure on the US bond market.

When the US and Japan intervene to buy yen, savvy investors see it as a desperate measure masking fundamental problems. This perception of rising risk without rising returns drives investment away, creating a feedback loop that can further weaken the currency.

The Bank of Japan's intervention is a defensive measure, not an offensive one. It aims to prevent an explosive, out-of-control yen depreciation (the 'right tail' risk) and buy time, hoping the underlying macro picture (like U.S. yields) eventually changes in its favor.

The Bank of Japan's intervention was not just about the yen, but a strategic move to "punt for risk parity"—to reduce volatility and calm markets. By strengthening the yen, they stabilized US Treasury rates, which in turn supported equities, revealing a tug-of-war between central banks seeking stability and traders seeking volatility.

The US is signaling a major shift from its long-standing 'King Dollar' policy. By being willing to devalue the dollar, it can strategically intervene in currency markets to bolster allies like Japan while simultaneously hurting economic adversaries like China by making US manufacturing more competitive.

Recent US Treasury actions, including unusually direct language in its currency report calling for Chinese Yuan appreciation and citing specific tariff threats, indicate a shift toward a more interventionist FX policy. This move away from a hands-off approach suggests the US may become a more active source of bilateral currency volatility.