Second Sino-Japanese War:
The Chinese had been prepared for the Japanese invasion - though evidently not enough. The Kwantung Army attempted to drive east to link up with the Imperial Japanese Army in Korea, though they were blocked at the Battle of Donggang.
Meanwhile, the fledgling National Revolutionary Navy attempted a blockade of the Liaodong Peninsula, aiming to force a surrender. Dalian was not wanted for economic reasons, but for political reasons.
So the decision was made. The Yellow Sea was mined. Japanese reinforcements were now blocked.
The Imperial Japanese Army now made several frantic movements to reach the encircled Kwantung Army. But it was useless. On July 4, 1925, the commander of the Kwantung Army finally surrendered to the NRA (not referring to the National Rifle Association, by the way).
Observers in Japan were shocked. How did China win such a large victory over them?
Troops began massing on both sides of the Chinese-Japanese border. Fortifications were rapidly put into place by both nations.
Unfortunately for Japan, the Koreans had been receiving arms for three or four years. With all the Japanese troops massed on the border, a rebellion began in Busan and spread rapidly to Hamhung.
When Japanese soldiers turned back to fight the rebels, Chinese troops flooded across the border in a huge attack (like they normally do). By September 15, mainland Korea was in the hands of China.
Korea, although the biggest front, was not the only front in the war. China, which had also gained Russian Sakhalin through Lend-Lease, found its portion of the island blockaded. But China’s coast was too big for the Japanese navy to completely blockade, and enough of the Chinese army survived on Sakhalin to hold their positions.
Meanwhile, on Taiwan, Chinese agents attempted to stir up rebellion (which was difficult).
Japan wasn’t on the defensive everywhere, however. Considering just how badly the Imperial Japanese Army had just been defeated, their prestige and standing in Japan had just been shattered. The Navy now led operations.
The Imperial Japanese Navy now attempted to take over China’s most successful city, believing that it could trade Shanghai (and surroundings) for Korea. After all, Nanjing was the capital.
Shanghai was taken, with huge civilian casualties, on February 14. There was a huge bloodletting, to the disapproval of all the major powers in the world. It didn’t help that there had always been a huge foreign population in China. For example, thousands of Americans lived in Shanghai, but after the Japanese takeover, only 382 remained. Chinese troops massed on the road to Nanjing, while the capital was relocated west to Wuhan. In the Battle of Suzhou, Japanese troops were finally pushed back.
Foreign aid poured into China after the Shanghai Massacre was made public. They were able to buy ships (though they didn’t have the sailors to man them). Regardless, the National Revolutionary Navy numbered several dozen battleships and hundreds of wooden sailing ships, in addition to several dozen new submarines.
These submarines were able to harass Japanese trade and destroy Japanese ships, while the sailing ships were generally used for troop transport and supply. By May 4, 1926, China had been able to clear the area around the Taiwan Strait. Within four days, around 2,000 troops had landed on the beaches of Taiwan to ‘liberate’ the island. Eight days later, the number had skyrocketed to 12,000, with all sorts of fishing boats from Fujian and Guangdong pitching in.
The Imperial Japanese Navy arrived in full force fairly quickly. The National Revolutionary Navy barely made it, but they escaped. They proved speedy enough to continue supplying the troops on Taiwan. China found itself supplying Japanese Communists, too, considering how the bulk of the Japanese navy was elsewhere.
Five months later later, Japan finally gave in to international pressure. In the Treaty of Seoul (November 7, 1926), Taiwan and Korea were all ceded to China, while the Chinese gave up their part of Sakhalin.
Meanwhile, everything above the Amur River was ceded back to Russia, too [1]. This was because Russian aid allowed Chinese troops on Sakhalin to hold out, allowing the exchange for Taiwan.
Korea became an independent member of ASEAN, and Taiwan was ‘integrated’ into China.
[1] I’ve decided that this is completely unfeasible and unlikely. So it’s only going to be Outer Manchuria south of the Amur.