Without the war in China the Japanese could deploy over a million men against the USSR; but if in 1941 they would have had difficulty operating in Transbaikalia, in 1937 it would have been nearly impossible. Accordingly, the IGHQ established the Great Khingan Range as the general western limit of advance. On the other side, the Soviet Army was a basket case with the purges.
Comparatively, the strength of the Red Army at the time was about 1,300,000, of which 370,000 men, 1,560 aircraft, 1,500 tanks, and 64-67 subs were located in the Far East (by Japanese estimates).
The Soviets judged the Japanese Army in February 1938 to have 1,200,000 men, 4,500 heavy guns, 1,800 tanks, and 2,000 aircraft. The Soviet troops in the Far East were generally the best they had, since the STAVKA regarded Japan as the most likely enemy before Hitler started causing trouble in Europe. However, it probably would have been difficult to maintain that quality over a protracted length of time as casualties, the diluting effects of mobilization, and Stalin's paranoia took their toll.
The Japanese War Plan for 1937 vis-a-vis the USSR was as follows (from JSOM vol. 1):
- D-30: Assemble striking forces at the Eastern Border of Primorye (Maritime Province)
- D to D+90 or 120: Destroy main body of Soviet Forces in Primorye, thereafter besieging Vladivostok with 3 divisions if it had not already fallen. Northern and Western fronts will fight delaying actions until main body of forces can regroup for pivots in this direction.
- Thereafter: Defeat the expected Soviet counteroffensives on the Northern and Western Fronts, especially around Qiqihar Plains, and begin an offensive toward the border. By the summer of 1938 a defensive perimeter running through Skovorodino and Hailar should be established, which will be held indefinitely.
The forces to be used were:
- D-Day: 21 divisions (15 against Primorye, 3 for holding actions in the north vs. Blagoveshchensk, 2 for holding actions in the west, and 1 in reserve)
For command of the air: 500 aircraft, of which 200 would initially be available in Manchuria
- D+15: 30 divisions
- D+30: 34 divisions
Additionally, smaller units (brigades, separate regiments, etc. were to be mobilized under direct Army control). Armor would be gathered into "mixed mechanized brigades" for "making attacks and exploiting results following breakthroughs in the border." Apparently the Japanese command intended to use their infantry and artillery to break through the Soviet defenses, while tanks were to be held in reserve. The IJA expected that 22 to 23 divisions plus smaller units would be available for the "Second Phase" push to the Khingan Mountains. This was expected to be a tough fight, since by that time it was anticipated that the Soviets would have brought in elite reinforcements from European Russia. As far as I can tell there is no direct prediction of the forces the Soviets would be capable of bringing to bear, but subsequent studies concluded that based on the capacities of the Trans-Siberian Railway they could eventually gather and support 55 to 60 divisions.