Colonial Scenarios of "Spain" if Portugal, Castile and Aragon were Unified

Japan seized a decisive victory in the Imjin War, successfully assimilated Korea, and never turned isolationist. Yu Sun-sin, the genius Korean admiral, was killed in battle against the Jurchen, or fell victim to false accusations of desertion and was executed, before the war. The Japanese were also able to make a few significant improvements to their shipbuilding technology thanks to trade and cultural exchanges with Greater Spain and the HRE. As a result, the Korean navy made a poor performance, the Japanese seized the upper hand in the naval war, and kept their supply lines secure throughout the conflict. The Japanese crushed Korean resistance, and were able to kill or capture the Korean royal family and court during their initial offensive drive, causing organized Korean resistance to collapse. Some degree of Korean resistance occasionally flared up for a few years, but was eventually suppressed. Korea was assimilated into the Japanese Empire by a mix of land redistribution to Japanese nobles, assimilation of Korean elites into Japanese ones, and cultural fusion.

Toyotomi Hideyoshi lived a few years longer ITTL and was able to establish an undisputed succession. Tokugawa Ieyasu was assassinated by a rogue Hojo samurai in the 1590s. Therefore, when the Tokugawa tried to challenge the supremacy of the Toyotomi in Japan, they were defeated without excessive difficulty. This, in combination with most daimyos being busy fighting or happily consolidating their new gains on the continent, secured undisputed Toyotomi control of the Shogunate.

The decline of the Ming dynasty got slightly accelerated in comparison to OTL, so it was already in its terminal phase by the end of the 16th century. Toyotomi Hideyoshi was able to form alliances with the seafaring Habsburg powers and the Jurchen leader Nurhaci against the Ming, in order to fulfill his own ambition of conquering Korea and China. He bargained Spain and the HRE's naval support for tolerance of Christians and free access of Spanish and Imperial merchants and missionaries in the Japanese Empire. He promised the Jurchen control of Mongolia, Manchuria, and Tibet. In his plans, the Japanese Emperor would become the overlord of Japan, Korea, and China, under the control of the Toyotomi as the power behind the throne.

When the Ming tried to contest Japanese conquest of Korea, they were defeated by a combination of samurai, Korean conscripts, and Jurchen raiders. They were pushed back all the way to Beijing, which fell. The Japanese also landed an expeditionary corps with Spanish/Imperial support and seized Nanjing. These defeats utterly discredited the Ming, whose power was already in severe decline. Many Chinese decided the Mandate of Heaven had been withdrawn, and China fell into chaos. Although Ming remnants kept some support across southern China for a few years, their power was broken when the last Ming emperor took his life after the fall of Nanjing.

The Japanese and the Jurchen started fighting for control of China soon after the collapse of the Ming. Eventually the Jurchen won in and entrenched their control of China proper and Mongolia, starting the Manchu dynasty. However the Japanese conquered and kept Greater Manchuria, Taiwan, and Hainan, which became subject to extensive Japanese-Korean colonization. Eventually, Manchu China and Toyotomi Japan-Korea accepted peace alongside these lines. Because of its alliance ties with the Habsburg powers, the Japanese Empire kept its borders open to trade and cultural exchanges with the West, ensuring its society developed at an equal pace with the European powers. It granted ongoing tolerance to Christians, ensuring a sizable portion of the Japanese-Korean population eventually turned Christian, although the Empire stayed religiously mixed. Manchu China, however, turned out isolationist much the same way as OTL.

The outward-looking, expansionist focus of the Japanese Empire and its colonization of Manchuria made Karafuto (OTL Sakhalin) and the Far East another natural target of Japanese expansionism. The Russians tried to contest this as part of their drive to expand across Siberia to the Pacific. However the Japanese-Koreans enjoyed a serious logistic advantage since the area was close to to their home bases and they could easily reach it by land or sea, whereas the Russians had to made a trek across all of Siberia through the Siberian rivers routes. As a result, Japan-Korea won all the colonial and border conflicts with Russia throughout the 17th century. By the end of that century, a peace treaty consolidated the Russian-Japanese border on the Lena River and Lake Baikal. Just like Greater Manchuria, Taiwan, and Hainan, Karafuto, the Trans-Baikal, and the other habitable portions of the Japanese Far East became subject to extensive Japanese-Korean colonization.