I am not sure where the confusion lies with Japan and war declaration.
“Each nation has different conditions under which it can declare war which are listed on their National Reference Sheet. In some cases, the nation only needs to reach its full Wartime Income level. In other cases there are more conditions to be met. Once you are able to declare war you may do so at any time, even during another player’s turn.”
This is the rule regarding declaring war. Now granted it is poorly word the line “more conditions” should read “different conditions”. However it clearly states the national reference sheet lists the condition to declare war. Japan’s are simple. They can declare war on any nation other than Italy and Germany. Using information from other National Reference sheets obscures the matter. For each nation only the core rules and its reference sheet apply.
Japan has no income criteria to declare war. There is no “governing principle” that a nation must be at full income to declare war, however to execute the war to it fullest extent certainly. Russia can declare on Japan in Jan 1939 if that is a strategically valuable option to the Russian. He must wage that war with 8 IPP per turn plus the d12 after July '39 but he may invade Manchuria and kill Japanese units to his fullest ability.
I will also add my voice in support of a version 1.3 (as well as a 1.4 and 1.5) to clean up different grey area and bad wording.
If you would like to see the extent of the shenanigans that the rules currently allow following an Axis War Dec.
German war dec a minor
Russia …. Declares war on Domincian Republic
Monroe Doctrine triggered
USA responds with war dec on Russia
USA goes to 76 IPC
Russia goes to 45
USA War dec Germany and Japan (probably attacked someone by Germany first attack)
UK War Decs Russia
Uk War Decs germany
France War Decs Russia
France War decs Germany
Germany completes invasion of minor thinking “WTF!!!”
Allies and Soviets all go full production and consume the Axis player before facing down in a final clash of the titans, most likely won by the Allies.