The point of who it helps is not really a big concern in my opinion. At least not in a game as big as G40. If They made the rules to make the defender the strongest, then the two forces would be standing right next to each other. if they made the rules so that the attacker is the strongest, then the two forces would deadzone each other.
there are so many rules and effects that help the attacker or defender, Order or casualties is probably one of the least important. More important include: unit stats, planes cannot land on taken terretory, planes needs carriers or airbases to defend a seazone, retreating gives you an extra round of production in your stack while removing one from your enemy, taking a capitol gives you all of the money, etc…
In the Pacific rulebook it states: “At the beginning of the game, Japan and China are at war. However, none of the other Allied powers…” thus implying that China is an Allied power, and Allied powers are unable to declare war on each other.
Yes, an act of war by Japan on UK or ANZAC(and maybe France?) puts them at war with the US.
It does not put them at war. It allows them to declare war. There is a rather large distinction, as Japan can end the turn in seazones with US ships (or attack seazones with both US and UK/Anzac ships, ignoring the US ships) that are still friendly to them until the US declares war on the combat move of its next turn.
Japan declaring war on France has no impact whatsoever. Japan can declare war on france and as long as they don’t attack FIC, they’ll continue to collect their National Objective. Even if they do attack FIC, japan only loses their objective. It does not permit the US to declare war.