Hmm, that is interesting. Still the Soviets won and their advance was ahead of schedule. I don't see the under strength Japanese army lasting more than as you implied six months.
The Soviet objective had been to encircle and destroy the Kwantung Army, at the time of the surrender they had failed to accomplish this or the occupation of any of the major cities of Manchuria. To quote from S.M. Shtemenko's "The Soviet General Staff at War" states, on page 354:
"To precipitate a real surrender and prevent unnecessary bloodshed, it was decided to land airborne forces at key points in the enemy's lines - Harbin, Kirin, Mukden, Changchun, and some other cities of Manchuria and Korea. After 17:00 hours on August 18th aircraft carrying the first group of 120 airborne troops under the command of Lieutenant-Colonel Zabelin took off from Horol and set course for Harbin. This force had the task of seizing the aerodrome and other important military installations, protecting the bridges on the Sungari and holding them until the main forces of the First Far Eastern Front arrived. With the first echelon of the airborne force was Major-General G.A. Shelakhov, the Front's deputy chief of staff, who had been appointed special representative of the Military Council. His duties were to present a surrender ultimatum to the command of the Japanese forces in Harbin and dictate its terms to them. We had no precise information about the situation of the city and the Soviet Consulate there. All we knew was that the main forces of the First Front of the Kwantung Army were falling back on Harbin after their defeat at Mutanchiang. They formed a very considerable force."
I should note this particular passage is about the First Area Army in particular, so even in the Soviet's judgement the forces they had engaged heavily were still a potent enemy. This is especially notable as well, given that despite the weakened posture of the Kwantung Army in 1945 compared to previous years, the forces at Mutanchiang inflicted equal losses upon the Soviets, destroyed hundreds of tanks and thereafter remained combat capable while conducting an orderly withdraw in the aftermath.
The Kwantung Army's planning at the time of the surrender was to withdraw into the Tunghua Redoubt, in Southern Manchuria near Korea; it is a mountainous area where the Japanese had prepared fortifications. Aiding this plan was the withdraw of the China Expeditionary Army into the coastal areas of China, done in order to shorten their own supply lines and allow for a better defense for prepared positions. This allowed IGHQ to detach six divisions and six brigades from the CEA, including the 3rd Tank Division, as reinforcements into Manchuria. All told, this represented about 180,000 to 200,000 Japanese soldiers from well trained, veteran formations would be joining the already 750,000 man Kwnatung Army into the redoubt, which was in a mountain zone with already prepared fortifications. So, all together, you're looking at about just under a million Japanese soldiers against around 1.5 million Soviets.
That the Japanese were consistently achieving a 1 for 1, or even better, ratio against both the Soviets and the Americans, this alone should be telling. Soviet medical records pre-invasion had projected at least 540,000 to 600,000 casualties, meaning that the observed battles by the time of the Japanese surrender indicated casualties were going to be much higher than thought. Adding to this issue was the very real supply constraints the Red Army was operating under.
According to Shtemenko, at the onset of operations STAVKA directed that the Kwantung Army be destroyed within 8 weeks or else the logistical situation would become "perilous". It's easy to see why they stated this, because the capacity of the Trans-Siberian Railway was limited to 13 million tons yearly in 1945 and of this only 9.3 million tons could be used for military needs; this is exactly why the Soviets requested MILEPOST deliveries from the United States. According to John R. Deane's "The Strange Alliance", on pages 263-264, the statistics provided by the Red Army to the United States as part of MILEPOST showed that they would be at a monthly deficit of 200,000 tons. Thus, the 1.25 million tons the U.S. provided in the three months between V-E Day and the Soviet invasion in August gave the Soviets a very limited window to achieve decisive results because after that it would become impossible. With official Soviet belligerency eliminating the ability of further MILEPOST shipments (The Japanese only allowed Soviet shipping through their waters while they were neutral) and the inability to expand rail capacity in the Far East (The Soviets started a project to do so Pre-War...and it took until 1984 to complete IOTL), we know the eight weeks limit is firm.
I should also add that eight weeks might be too generous.
Their exploitation force was bingo on fuel and thus immobile by day three of combat operations:
"Soviet sources do recognize severe short comings in their own logistical planning. The available supply transports were too few to cope with the demand. The road conditions were poor and, together with the rainy weather, caused severe delays in resupply operations. Estimates of fuel requirements were proved to be totally wrong. This severely affected the 6th Guards Tank Army in western Manchuria. This mobile army which was to operate deep behind enemy lines as an operational manoeuvre group (oMG) was in fact out of fuel already on the third day of the operation. It had to be resupplied with emergency air transportation of fuel. one peculiar fact is that the Soviet logistical planning relied heavily on the unrealistic assumption of using enemy railroads for troop and sup ply transports in Manchuria. This raises serious questions of the quality of the Soviet logistical planning. Another explanation is that the Soviet attack actually began before all necessary logistical preparations were in place. However, by launching an attack at an early stage it probably contributed to the creation of surprise."
That wouldn't defeat the Americans though? They'd just bring in more troops from Europe. As for public opinion, I imagine at first the public would be shocked but then out of rage would demand the US continue the war until Japan would crushed into dust.*
*this is the inevitable outcome, even if Olympic had been beaten back, the US would have just bombed again, and then done it later.
I suppose that would continue the war, and that yes eventually the American public might have wished to simply make peace with Japan, as would elements of the government.
So...that means the war continues into the later forties?
The U.S. landing force was to be, in total, about 700,000 men. Contrary to what U.S. intelligence thought and to which Major Arens points out was the case, the IJA had 900,000 men in position with the expectation of 90,000 reinforcements, exclusive of any IJN, IJAAF or civilian militias to be used. As Iwo Jima and Okinawa had showed, the Japanese had by this point in the war refined their tactics to routinely achieve 1:1 or even better ratios of casualties. I'll cite some casualty projections:
- In a letter to General Curtis LeMay when LeMay assumed command of the B-29 force on Guam, General Lauris Norstad told LeMay that if an invasion took place, it would cost the US "half a million" dead.
- In July MacArthur's Intelligence Chief, Maj. Gen. Charles A. Willoughby, warned of between 210,000 and 280,000 battle casualties in the push to the "stop line" one-third of the way up Kyushu. Even when rounded down to a conservative 200,000, this figure implied a total of nearly 500,000 all-causes losses, of whom perhaps 50,000 might return to duty after light to moderate care.
- n the spring of 1945, the Army Service Forces under Lt. Gen. Brehon B. Somervell was working under a figure of "approximately" 720,000 for the projected replacements needed for "dead and evacuated wounded" through December 31, 1946, which was for the whole invasion including Honshu. These figures are for Army and Army Air Force personnel only, and do not include replacements needed for the Navy and Marine Corps.
- A study done for Secretary of War Henry Stimson's staff by William Shockley estimated that invading Japan would cost 1.7–4 million American casualties, including 400,000–800,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan.
- The US Sixth Army, the formation tasked with carrying out the major land fighting on Kyushu, estimated a figure of 394,859 casualties serious enough to be permanently removed from unit roll calls during the first 120 days on Kyushu, barely enough to avoid outstripping the planned replacement stream.
Operating under the assumption of just 300,000 to 500,000 defenders, 6th Army was already projecting the replacement stream would be brought to the point of being unable to cope; as already pointed out, that's major, as actual defenders were over 900,000. As for troops from Europe in terms of formations, no existing planning was conducted to do such meaning there was none in the "replacement stream" to be used. To do so would require the Army to halt their entire demobilization plan-politically unpopular-and then reorganize their forces in Europe and then begin the long running transfer of the same to the Pacific. This would take, on average, about three months or so. Operation Olympic would be, as Major Arens points out, decided long before then.