One must get reinforcements into the mainland of Asia, yet at the beginning of the game the territories on the mainland are relatively safe. America will build up solely on its navy at first, since it will see your navy as a prominent threat (the only threat to America at the beginning is the Japanese navy and the japanese surrounding China will probably be brushed aside as the cost of an IC is expensive, and it would take several turns to get american transports to china) anyway, as long as you play yer cards right with germany, Russia will focus on germany until forced to juggle with germany and japan. Germany must take Karellia, as it has an IC and will keep Russia spread out thin between Karellia and Caucusus, allowing the smart German player to take both eventually. Japan has no major threat on the mainland at first, tho it is very wise to do away with the americans in china and the british in india, to increase income and be able to focus on the major purpose of occupying the mainland: RUSSIA
as long as the japanese navy is a match for the American navy, land troops in mainland asia can be built moderately, why build transports when one can quickly build an IC and build reinforcements in RUssia and still keep the americans in check, why send your island troops to mainland? instead keep them their so that the americans will be at bay, dont let the americans go island hopping
so in essence, in my opinion one SHOULD build an IC in manchuria and do the stuff i just described, of course the dice and the opponent always influence the outcome, strategy and thought is key