10 Inf / 1 MInf / 1 Tank / 1 Arty / 5 Fighters / 4 T Bombers and 2 Heavy Bombers all together is what Japan can bring to bare Turn 1. If I chose to leave 2-3 of the Inf in Manchuria to protect my flank that resolves the Mongolian threat. I have to land the fighters in a friendly space so they come back as well. Defending fighters are pretty good against Infantry.
If I were playing Japan and my Russian opponent didn’t stack his Inf together and I wanted to push that direction I would push into Amur steam rolling anything there, if he left Inf in Siberia, they’re gone as well in a seperate attack with (if I felt it was prudent) a bombardment in addition to the amphibious assault. That’s 2 IPC’s I gain first turn and Russia loses at least 6 Inf depending on how they positioned. They get Mongolia with it’s 6 Inf (in deposits of 2, 2, 1, and 1 it’ll be very helpful…) Russia can’t hope to gain anything back by attacking so their next turn is spend either consolidating their units, doing some sort of delaying tactic, and if they want they could send the Mongolian Inf into China. The closest reinforcements they have are 3 turns away(in Moscow with their 1 tank and 1 Mech Inf that should turn the tide right?) and 2 turns away by air (also from Moscow 1 fighter and 1 tac bomber). Those units are not only required in the west should Germany attack, but also aren’t nearly enough to push the Japanese away.
J1 - Amur and Siberia (or Soviet Far East if the Russian player didn’t put Inf in Siberia) 2 IPC’s and the loss of at least 6 Inf to the Russians.
J2 - Siberia (or Soviet Far East if I amphibed here last turn), Sakha, Buryatia and possibly Buyant-Uhaa through Jehol and Inf left in Manchuria (Depending on what Russia decided to do with them)
That’s 3 more IPC’s and again depending on how the Russian player positioned their forces another 6 or 12 maybe 13 if the 1 Inf from Ulaanbaatar was moved and 2 AA Guns.
And now there is 3 lets say 3 more in in Yenisey that was moved from Mongolian Dzavhan Olgiy.
J3 - Yakut, the Tank and the Mech Inf in Moscow could have arrived if the Russian player happened to have moved them this way BEFORE the Japs attacked turn 1 so lets say they’re there, and the plane could be here as well. So this is the only battle that could hurt Japan.
So this battle would be the Russians with probably 3 Inf, 1 Tank, 1 Mech Inf and lets say the plane. Japan could attack with 1 Tank, 1 Mech Inf, 4 Fighters, 4 Tac Bombers and 2 bombers.
Japan probably wouldn’t need to commit so much aircraft to this because A) it’s unlikely the tank and MInf were moved east first turn, in fact they were probably moved west towards Germany which would make it turn 5 before they could arrive. B) It’s also not too likely they would have moved the plane either, it’s a suicide mission and the plane is needed in the west.
Japan would probably lose the Mech Inf and 1 or 2 aircraft, Russia, loses what’s in the territory, and now there is nothing between the Japanese and Moscow (in terms of units in territories, things would be purchased and placed in Moscow) but it’s also almost assured that Germany has attacked Russia this turn or the previous turn preemptively so they’re going to be a little busy.
You tell me how a ‘solid Russian player’ prevents it, or doesn’t take serious Economic damage because of it.