I find that people new to Japan and want to learn how to play it will do J1 DOW, because it can be relatively strait forward. You take islands, you make money, the end.
Experienced players tend to do J2, unless G1 went really bad for them… in which case they will do J1 and try to bang out a win that way.
The reason experienced players lean towards a J2 is because you can get diced doing a J1 DOW, Like the bship can get 2 hits, Phil can sometimes score 4 hits. Kwangtung can be annoying with snake eyes, yunnan can get 4 hits and make you sad a little.
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In a low luck game, J1 DOW is the most optimal decision. All your attacks go smoothly, you need only 2 fighters on Japan to defend. Everything perfectly calculated. The bship dies in one round of combat hits the cruiser 2/3 games and the other 1/3 you have a cruiser alive (yay). Everything is good.
J1 DOW low luck games, all day every day. I do not care if Russia supports china or what have you. I do it because I want money and I want India making less money sooner. I am taking India rounds 4-6 depending on what juicy attacks the allies want to give me or not give me.
Sometimes you have to make a choice to be defensive and keep USA from getting a foothold in Korea / Japan sea zone… or to just take India and kick him out later. Like those are legitimate decisions you face and you can go either way on that one. Personally I prefer to knock out Calcutta and deal with USA later (bring german bombers to the Pacific this prevents those lone destroyers from blocking your entire fleet).
Yes German bombers in the Pacific are critical, bring them over after Germany takes russia unless something went wrong in Europe and you are betting on Japan winning… then by all means bring bombers over sooner than later.
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Funny story, I attacked Russia with 20 tanks and missed 17 of them… Russians hit almost all their dice… I only had one option left… win in the Pacific within the next two rounds… had 9 german bombers in China after that failed attack… 9 German bombers was the difference between me taking Hawaii and me losing the game. He could not block my naval. I could not be stopped and the 9 german bombers suicide attacked before Japan went ALL IN. I sunk his naval barely and took Hawaii.
Good times. If I had more time I would out maneuver him and go Carolines to threaten New South Wales and Hawaii.
Another game I had German bombers defending the Kiangsu because I took New South Wales after Calcutta and I needed to hold all the VCs. Rolling 6 bombers and 2 japan fighters against 8 usa attackers… I DEFENDED! He was so mad.