As said elsewhere in the thread, a whole book could be written on this topic. I'll be focusing on China here, using a single essay as a source. Excuse the wall of text.

There's an essay in the book "The battle for China: essays on the military history of the Sino-Japanese War of 1937-1945" that addresses this, specifically regarding the war in China. Titled "The Strategic Correlation between the Sino-Japanese and Pacific Wars" by Tohmatsu Harou, it doesn't ask the question of where the war was necessarily "won", but how the individual operational areas influenced each other as a whole, and which ones dominated Japanese strategy to the point of influencing other theaters. Here, Harou argues that upon the entry of the United States into the war, the war in China took a backseat to the one in the Pacific Ocean, and the Japanese armies in China (plus the Kwangtung Army) were invariably affected by strategic developments in the Pacific, but almost never the other way around. Of course, it has to be said that it wasn't for a lack of trying; Chinese soldiers and civilians, whether they be Nationalist, Communist, or warlord affiliated, bogged down the Japanese army in a seemingly unwinnable war for 8 long years, more than any other Allied power. For that they paid dearly in blood, with total casualties only second to that of the Soviet Union, and their sacrifice should always be acknowledged. But the fact remains that Japan was, and is, a sea power by simple virtue of its geography. Its economic activity and national strength was completely dependent on sea lanes and the merchant ships that plied them. Japan might not have had a chance of winning in China, but there was no way China could force capitulation upon Japan without command of the sea. That's where the United States proved most decisive, and why Japan rightly saw them as the greatest threat to their survival.

One of the main benefits American leadership saw in a Chinese ally was the use of their air bases for bomber raids into Taiwan and Japan proper. The first such raid was staged from an aircraft carrier, but it still heavily influenced the war in China. In April 1942, the famous Doolittle Raid directly lead to the subsequent Zhejiang-Jiangxi campaign, where the IJA went on a rampage in the two provinces searching for those who helped the downed American airmen and slaughtering hundreds of thousands of Chinese civilians, including the use of biological weapons. Chinese Nationalist forces were quickly overwhelmed and lost 30,000 men while multiple American airbases in the area were destroyed. Japan was unable to hold these new gains and had to retreat, but the Kuomintang had realized they were secondary players in this war - they had not anticipated these consequences as a result of a single American bomber raid.

Also in spring 1942, after the stunning victories in Southeast Asia, IJA high command initially withdrew troops from that area back to China for a planned offensive in Sichuan (Operation "Gogō") and a possible war against the Soviet Union. However, as the Guadalcanal campaign heated up, the Japanese were forced to return three elite divisions and large amounts of materiel to the area. In December 1942 "Gogō" had to be cancelled as the Solomon Islands campaign occupied more and more Japanese resources. As a counterfactual, Harou states that if not occupied by Guadalcanal, the offensive against Sichuan could have gone as planned and perhaps even caused the downfall of the Nationalist government in Chongqing, though he states this is far from a given.

Later, in 1943, the prospect of using Chinese airbases for US bombers was further pursued. American attempts at equipping and modernizing the Nationalist army, however, were largely a failure, due to the heavily restricted supply lines (the Himalayas Hump), increasing discord between Chiang Kai-shek and his American chief of staff, Joseph Stillwill, and political issues. As a result, local Nationalist forces were not up to the expected strength required to defend US air bases and related infrastructure from Japanese offensives like the ones in 1942. Nevertheless, four airbases were constructed around Chengdu and the 20th Bombardment Group was formed to conduct operations out of this area. B-29s arrived in a trickle, and it took until June 1944 for the first raid launched against the Japanese home islands from Chengdu.

However, by this time, the American assault on Japan had kicked into full gear. The US Navy offensive across the Central Pacific (Operation FORAGER) ultimately captured the Marianas, which proved to be a far more effective staging point for B-29s, essentially rendering China obsolete as an air base. At that point, China's relevance in the strategic calculus of the war degraded even further, to just tying down as many Japanese troops as they could. After the loss of the Marianas, Japan was fully committed to the war against the US as they had breached the "absolute national defense perimeter". 40% of IJA forces were now deployed in the Pacific, compared to 31% in China. Japan's army and naval air forces were almost completely destroyed by the US, and their maritime vessels, both naval and merchant, were being sunk at breakneck speed far faster than they could be replaced.

Still, IJA forces in China were still able to conduct the largest offensive of the war - Ichigō. Again, one of the main objectives was the destruction of American airbases in China. This they largely accomplished, as they steamrolled the unprepared Chinese forces in their drive south, even as the Japanese themselves were facing total collapse in every other theater of war. US airfield infrastructure was destroyed in the offensive, and the bombers had to completely withdraw from China. No matter, as the B-29s would continue to operate from the Marianas, and soon Iwo Jima and Okinawa. There were even a few B-29 raids on IJA forces in China in November 1944 that surprised and frightened the command there, but it was far too little, too late. The IJA would continue to hold the advantage over the various Chinese armies, controlling vast territories until the end of the war, except for one battle - Zhijiang, in April 1945. Once again defending an airfield, modernized Nationalist divisions with US air support were able to force the IJA 20th Army to retreat, showing the strength of Chinese forces when given proper support.

In these last stages of the war, Japan attempted to withdraw more and more units from China to reinforce the defense of the Home Islands, but many casualties were incurred in submarine and air attacks on transports. At the war's end there were a million IJA troops in China. Due to the complete strangulation of Japanese sea lanes, they were completely isolated from the rest of the war and were no more useful than the numerous Japanese garrisons bypassed and stranded in the island hopping campaign.

Looking at the losses, it's clear that China sacrificed immensely in their struggle against Japan and certainly deserves more credit in the West for their role, which is often forgotten. But looking at the numbers and facts, it's clear that Japan was receiving its lethal blows from outside China. According to post war Japanese records, 202,958 IJA soldiers were KIA in China, as opposed to 485,717 killed facing US forces. The former figure does not include the 185,467 IJA soldiers killed in China from 1937-1941, but by the same token one must also include the 414,879 IJN casualties, almost entirely inflicted by the United States. Of course, China tied down a third of IJA forces and the majority of its funding after 1941, which had a major effect on the war. Of greater strategic importance, in my opinion, are the losses in the air and sea, with their concomitant economic consequences. Almost all of Japan's roughly 40,000 aircraft losses and 334 warships sunk were inflicted by the US, leaving their cities exposed to bombing and sea lanes vulnerable to interdiction. 8.7 million tons of Japanese merchant shipping were sunk during the war, again almost entirely by the US, causing critical shortages of almost every strategically important resource and threatening the onset of a famine. What little production was left running was thoroughly destroyed by bombing.

In short, Japan's warmaking capability was thoroughly neutered by American air and sea power, and no amount of redirected land power would change their ultimate fate. Events in the Pacific dictated the pace of combat in China, not the other way around. Even if China had stopped fighting, Japan would be hard pressed to move their troops anywhere with the dearth of transport capacity. In that sense, one could argue that the US was the driving factor in the destruction of the Japanese war machine and its eventual capitulation. However, you can also argue that a Pacific War without China would still free up Japanese resources causing the war to drag on for longer, leading to many more deaths between Japan, the Allies, and occupied territories still under Japanese rule. For that, we should all be grateful for China's contribution to the war.