Some understanding of the nature and scope of the war in China is needed here. It appears that some view it as a minor sideshow, in which the Japanese could basically ignore or reduce the scales of deployment in with no ill-effects.
The Chinese war did ultimately develop into a sideshow, but only because the Japanese widened the scale of the conflict, because they found they could not win there. They believed that continued Chinese resistance was due to the support being provided by the western allies the Soviets, and Germany (yep, thats right, the Germans were gunning for a Chinese victory against the Japanese, this continued even after the signing of the Tripartite Pact, just proving, incidentally that agreements with the Nazis were worthless)
Another little political reality that sort of erodes the notion that Japan and Germany might somehow co-operate over Russia was that the 1939 non-aggression pact between Germany and Russia was viewed with great anger on the part of the Japanese, and led directly to some major offensives in China in which the Japanese were heavily defeated ). Under those circumstances the Japanese would never have been properly informed of an impending attack (as indeed they werent.....officialy at least. The Japanese high command was as surprised as everyone by the German attack). So, this notion of German -Japanese coperation before 1941 is basically navel gazing. It was never going to happen...
Anyway, to return to the issue of China, the Japanese are often assessed as having suffered about 1.5 million army casualties. However this is not correct, because it only measures the losses from 1941 to 45. In fact, Japan had been waging war in China, more or less continuously since 1931. Since 1937, the fighting had intensified, and in total Japanese military deaths in the period 1937-41 (excluding operatiuons outside China) amounted to 1.6 million additional losses(they had lost about 200000 before 1937). Thus the casualties 1931-41 amounted to 1.8 million, in addition to those lost after 1941. The Japanese army had suffered approximately 50% casualty rates amongst its frontline formations, which of course had been replaced, but it was these causalty rates that influenced the Japanese to decide to attack the west directly. That (or at least the relaisation that they could not win under the current conditions) plus the threat of the Allied oil embargo (which would have brought them to their knees in less than 10 months unless they did something about it) were the reasons why the war was expanded by the Japanese. Whilst China became a backwater eventually, in 1937-41 it was not viewed that way by anybody.
So why did the Japanese come to believe they could not win, and in fact were facing ultimate defeat if things remained the same as they were. Its not that they did not try to seek a result in China, but from 1939-41 they actually suffered a series of quite heavy defeatsas they stepped up their efforts to achieve a decisive result in the theatre.
The following is a list of the major Battles fought in China from 1937-41. It shows that the battles fought in 1937-38 were uniformly Japanese victories, whilst those fought after 1939 were uniformly Japanese defeats. I have marked the clear Chinese victories by the words (China) after the relevant battle
Battle of Lugou Bridge (Marco Polo Bridge Incident) July 1937 (Jpn)
Beiping-Tianjin July 1937
Chahar August 1937
Battle of Shanghai August 1937
Beiping–Hankou August 1937
Tianjin–Pukou August 1937
Taiyuan September 1937
Battle of Pingxingguan September 1937 (China)
Battle of Xinkou September 1937
Battle of Nanjing December 1937
Battle of Xuzhou December 1937
Battle of Taierzhuang March 1938 (China))
Northern and Eastern Honan 1938 January 1938
Battle of Lanfeng May 1938 (China)
Xiamen May 1938
Battle of Wuhan June 1938
Battle of Wanjialing (China)
Guangdong October 1938
Hainan Island February 1939
Battle of Nanchang March 1939
Battle of Xiushui River March 1939
Battle of Suixian-Zaoyang May 1939 (China)
Shantou June 1939
Battle of Changsha (1939) September 1939 (China)
Battle of South Guangxi November 1939 (China)
Battle of Kunlun Pass December 1939 (China)
1939-40 Winter Offensive November 1939
Battle of Wuyuan March 1940 (China)
Battle of Zaoyang-Yichang May 1940 (China)
Hundred Regiments Offensive August 1940 (China)
Vietnam Expedition September 1940
Central Hupei November 1940 (China)
Battle of South Henan January 1941 (China)
Western Hopei March 1941 (China)
Battle of Shanggao March 1941 (China)
Battle of South Shanxi May 1941
Battle of Changsha (1941) September 1941(China)
The biggest defeat for the Japanese was at 100 regiments, where Japanese casualties ran at 35-50000 men.
Ther are several important observations to be made from all this. Firstly, China was not a walkover. The KMT decreased their levels of activity after 12.41 for the obvious reason that they expected the US to win the war for them, plus their ability to wage offensive war had been severely curtailed after the Burma Road had been closed. If the Soviets had been attacked in 1941, the Chinese can be expected to continue, and intensify their actions, because they could now supplement their re-equipment via the Silk Road through Kazakhstan. To believe that the the Japanese could decrease their level of committment in China, leaving the collaboratuionist armies to hold more of the line, is sheer wishful thinking. The more likley outcome is a renewed effort by the Chinese, particulalry the CCP formations and therefore an increased force level for the Japanese, just to hold the current line. They could try and withdraw from China to Manchuria, but this would only allow the Chinese to advance in their wake, and permit a link up with the British forces, which one could expect to be advancing up through Vietnam.
And this talk of 2000 Japanese tanks does not take into account the losses that had been suffered in China since 1937, or the continued need to provide tank support there as well. The Japanese had lost somewhere in the vicinity of 1000-1200 tanks since 1937 (I can be no more accurate than that).
So this idea of raising another 50 divisions and another 1000 aircraft needs to be balanced against the fact that the Japanese were locked in a bitter struggle that they appeared to be losing, were not aware of the impending german attack (and were never going to be told either),
The other furphy was this idea that the japanese were going to win hearts and minds. Whilst as at December 1941 they had raised collaborationist forces in China totalling 669000 men, these were very rarely risked in battle, because they were so unreliable. 50% desertion rates were not unheard of, with the wepons they carried usually ending up in KMT or CCP hands. After the rape of Nanking, and the deaths of over 20 million Chinese citizens, the sparse populations in Far Eastern Russia were under no illusions as to what they might expect under a Japanese occupation. After the initial greetings, the true intentions of the nazis had soured any real hopes of "turning" the Russian population (though torture, racism and brainwashing did have its successes). This was never going to work in the Far East any better than in the West with the Russian population , in fact past Japanese abuses would probably have returned even less than it did for the germans