USD/JPY briefly surged through the 155.00 level on Wednesday, touching 155.05 after a break above 154.50 triggered a wave of sizeable stop-loss buying. Momentum accelerated as stops layered above 155.00 were flushed out, before fresh offers just beyond the figure finally slowed the move.The jump has refocused attention on Japan’s tolerance for renewed yen weakness. Finance Minister Katayama warned that the disadvantages of a weak yen now outweigh its benefits, adding to the familiar Ministry of Finance mantra about “disorderly” and “speculative” moves — language that typically signals heightened discomfort.With USD/JPY pressing into the mid-155s, traders are increasingly alert to the risk of intervention. Past episodes of verbal warnings have tended to cluster around these levels, and history shows the MoF’s trigger point has less to do with a specific line in the sand and more to do with speed, scale and disorderliness of the move.Beyond the technical dynamics, attention is turning to the MoF’s strategic calculus. Officials are fully aware that fundamentals — interest rate differentials, inflation trajectories and relative monetary paths — do not favour the yen. Any intervention is therefore unlikely to reverse the broader trend. The realistic aim would be to slow the ascent, punish crowded yen-short positions and reintroduce two-way risk to a market that increasingly feels one-directional.That makes timing critical. The MoF is unlikely to intervene during peak liquidity, when the impact would be muted. Instead, a thin-market window would deliver maximum effectiveness, especially if positioning is stretched or speculative longs are leaning heavily. With that in mind, the Thanksgiving period stands out as a plausible window — thinner global participation, lighter liquidity, and a backdrop ripe for a sharp, attention-grabbing move. This article was written by Eamonn Sheridan at investinglive.com.