In all frankness, I have never seen KJF work. Yes, I have seen Japan be completely hosed, with income down to the teens. Despite this, the US still has to spend money to keep Japan down and is unable to dedicate 100% of its resources to Europe.
Admittedly, I have only played with a few dozen players. Maybe there’s a good KJF out there that I have yet to see.
It is hard to script out KJF because Japan has soo many options, but here are a few random thoughts:
First, Japan has other income options that don’t depend on its navy. Crushing China early and taking Russian territory helps Germany and Japan while impeding Russia.
Second, it does not take that much US income to give the UK and Russia the edge over Germany and Italy.
Third, it takes almost the entire US income to overcome Japan’s navy.
Fourth, it is completely up to Japan when to go to war with the US.
All of these factors lead me to believe that KJF as a primary plan can’t work because it fundamentally assumes that your opponent is going to do certain things a certain way.
If you know your opponent always does a J1 attack you can certainly plan a response for that, but in terms of strategy that works against every opponent you need more tools than that in the toolbox.
Marsh