It is not something japan has to deal with right away. It is a solid move by the allies. japan’s major focus is india and picking up those vcs.
round 1 amur = japan kills it no prob.
round 2 amur-> round 3 korea = japan ignores it. still pushes for cash islands and calcutta. optimum / standard play is not really affected by what you do over there.
the units that can’t make it to calcutta in time for the showdown are generally reserved for killing china. how japan does his setup over there is generally case by case.
J1 and J2 should be standardized at this point. I don’t see any reason to not buy transports and an artillery round 1. Don’t see any reason to not get the spice islands on J2.
The other option for Japan is to start war J1. again I would still buy transports… but I have seen some fairly interesting pacific campaigns resulting from J1 war. In fact it is usually germany who gets mad at you for doing this, because the allies have no counter attack for you on the first round. round 2 it is a little trickier because your carriers are in hawaii seazone. hopefully your bship was the only thing tilted no lucky two hit BS.
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on a side note, if those infantry ever go to manchuria, they will die. the kwangsi air base is pretty standardized now, I seldom see players not purchase one there or for kiangsi (alternatively, also kiangsi reaches korea so I might be careful and eye out how many xports are in range).