Japan.
Japan is a nation in East Asia, and one of the dominant powers in that region. It is an ancient monarchy, the oldest in the modern world.
Japan's first treaty in modern times was the Yamagata-Macmillan Treaty with Great Britain in 1900. Japan had already emerged from its earlier isolation by that point, however, as evidenced by the reference to ambassadors to Mexico and Russia. It is probable that there had already been a reform of the government by that point as well, probably with some sort of parliamentary system. Sobel mentions a Premier. The Emperor at that time was known as Meiji, a posthumous title meaning Enlightened Rule.
Japan probably emerged from its isolation gradually. The Yamagata-Macmillan Treaty was Japan's first treaty with a Western power, indicating that it avoided the Unequal Treaties imposed on China. Sobel does not state what motivated Japan to engage with the outside world, but a possible motivation would be Russian expansionism threatening the northern islands of Japan, making it clear that permanent isolation was not a feasible policy. The details of its emergence is unknown, beyond the fact that an Imperial restoration took place, and that the capital was moved from Kyōto to the former Edo, subsequently renamed Tōkyō. Edo/Tokyo had been the administrative center during Japan's isolationist period, but the official capital remained where the Emperor was located, in Kyoto.
Japan is referred to as having suzerainty over several territories, including Taiwan and Indo-China. It seems plausible that Japan treated its neighbors in a manner more aligned with traditional Asian relationships of suzerain and tributary than western-style annexation or colonization. It may be that Japan viewed itself as a defender of East Asia, but the exact details of its relationship with other nations in the area are unclear. Japan had influence in China, and seems to have been one of the nations that controlled portions of China. However, there doesn't seem to have been an antagonistic relationship, as China and Japan were allied during the Global War. Perhaps Japan's position within China was more of a protector than a colonizer.
In the 1920s, Japan had a ban on Kramer Associates operating within their nation, but by the time of the Global War, that ban appears to have been lifted, as evidenced by a reference to K.A. associates within Japan warning the Japanese government of Mexico's plans.
During the Global War, Japan suffered a number of initial losses, and the cities of Tokyo and Nagasaki were heavily bombed.
After the war, Japan emerged as suzerain over Siberia, and, at least in appearance, the major power in East Asia. Taiwan was taken by Kramer Associates. Sobel does not specifically mention whether Japan retained nominal suzerainty over Taiwan nor the details of KA's takeover. But he does state that Taiwan's government remained formally separate from Kramer Associates, so its takeover was probably more along the lines of using its influence to install a friendly government, whether through democratic elections or by a coup. An election would be more KA's style. At any rate, Japan emerged from the war economically dependent on KA for some time. Sobel states that Japan "was dependent on Kramer Associates and would be for many years" and that in the late 1950s the company was "persistently but slowly turning Japan and Australia into economic colonies". The use of the past tense suggests that by 1971, Japan was beginning to emerge from KA domination. Japan remains an ally of Great Britain, but relations are strained between Japan and Australia, which may some day force Britain to choose between Australia and Japan in the event of a clash between the two.