There are tons of people for the J1 attack. However we have played our last 6 games with Japan attacking on the 4th round. Even though the UK gets extra money from the islands it doesn’t compare to the USA’s bonus money. last 6 games Japan has taken India in round 5 every time.
Now we have played multiple games with the J1 attack and are about 50/50 on who wins.
I believe you are correct. The “bonus” 30 the US gets when it’s attacked early is to compensate for not having extra money while “at war during collect income”, for example:
The US is attacked turn 2, gets 30 IPCs immediately so they have that to spend when it becomes their turn. If this were NOT the case, then the US would only have their initial 30 IPCs to spend during “war time” which wouldn’t make sense.