Greetings! This is certainly an interesting question and it puts forth an interesting proposition for us to consider. The rise of nationalism and imperialism in Japan's Showa period is certainly something that continues to fascinate historians of the 20th century, and perhaps if we search hard enough, we can find some links to the rise in nationalist feelings at the "Unequal Treaties" which helped kick off the Meiji Restoration and subsequent modernization efforts. Let's see what we can uncover, and if the Unequal Treaties were indeed part of the descent into the "dark valley" of the 1930s.

Turn of the Century, Triumph of Modernity?

To begin this response, let us turn to a rather interesting (if somewhat dramatic) quote by Christopher Goto-Jones regarding the impact of the Unequal Treaties, the first of which was (as OP has identified in their explanation), the Convention of Kanagawa.

"By 1858, the so-called Unequal Treaties regime was firmly in place: without a single shot being fired Japan found itself in a similar position to China after the Opium Wars...Japan had lost control of its tariffs, had opened its borders to trade and commerce with the West, and had even granted the privilege of extra-territoriality to the Western powers. Rather than being justified by military defeat however, these measures were imposed on Japan on the basis that it was not an equal member of international society - it was not a modern, industrial, constitutional polity. As we will see, this humiliation was itself a powerful force fueling the development of a strong sense of nationalism in late 19th-century Japan, as well as a key factor driving the revolution to come. At all costs, Japan sought to end the Unequal Treaties.

Now without going too much into Pre-Meiji Japan and the bakufu governments of the Tokugawa shogunate, it is important to note here that the humiliation of Japan at the hands of the Western powers had not damaged a pre-existing sense of national pride. The fractured natured of Japanese society with its feudal hierarchy and the perceived "backwardness" (at least by the West) of its economic and sociopolitical systems meant that the Unequal Treaties actually contributed to the rise of a Japanese nationalism. Andrew Gordon on this interesting effect (italics are from his writing):

"Yet it would be misleading to conclude simply that these treaties trampled a preexisting national pride and sovereignty. Rather, from the early 1800s through the 1860s, the very process of dealing with the pushy barbarians created modern Japanese nationalism. Among shogunal officials, in daimyo castles, and in the private academies where politically concerned samurai debated history and policy, a new conception took hold of “Japan” as a single nation, to be defended and governed as such."

Over the course of the Meiji Restoration and the subsequent reforms to Japan, the quest for a "Japanese" sense of identity remained a key sticking point for government officials, intellectuals, and individuals who argued about what citizens of this new national polity held in common. Even by the 1890s, the rapid transformation, Westernization, and modernizaton of Japanese national systems had yet to generate a unified idea of what it meant to be a "Japanese" person. In one of many forthcoming paradoxes of Japan's modern history, this search for a modern identity in a time of great change would soon yield a curious result: the idea that adherence to traditional Japanese values was a key part of the individual (chief among them: loyalty to the Emperor).

Thus then, at the turn of the 20th century, we have a rising Japan seeking to assert its position in Asia and shake off the humiliation that the Western powers had levied upon it almost half a century earlier. This new, modern, Japan would soon be seeking for itself a place among the great powers and empires of the age, as the Asian hegemon.

The Sun Rises

Even before the outbreak of the First World War, Japan had proven itself in several major conflicts. The first of these (for our case) was the First Sino-Japanese War of 1894-1895, in which the Rising Sun proved far superior with its new Westernized military and navy over the forces of the Chinese Dragon which it had long been a rival power of. The Treaty of Shimonoseki confirmed Japanese interests in Korea (which had come under Japan's sphere of influence since the Japan-Korea Treaty of 1876), claimed the island of Taiwan (then known as Formosa), as well as the Liaodong peninsula on the Chinese mainland.

For several years prior to the war, the Japanese government had been trying repeatedly to seek revisions to the Unequal Treaties, dispatching a third Mission to the United States as early as 1876, and other European nations also hosted Japanese missions with similar objectives. After the Sino-Japanese War and the success of Japan, the Western powers (many of whom had initially hedged their bets on a Chinese victory) were quick to applaud the Japanese for their modernization, and by 19897 all of the Unequal Treaties had been ended (though full tariff control to the Japanese was not given until 1911). The British were the first, rather interestingly, to terminate their Treaty (though admittedly in various "stages"), signing a new one in 1894 (just two weeks before the Sino-Japanese War). In April of 1895, with the ink on the Treaty of Shimonoseki barely dry, the London Times quoted British admiral and parliament member Lord Charles Beresford on Japan's rise to the status of a global power:

"Japan has within 40 years gone through the various administrative phases that occupied England about 800 years and Rome about 600, and I am loath to say that anything is impossible with her."

For the Japanese however, the West was not always so cooperative during this period of its rise. Shortly after the Treaty of Shimonoseki had been signed, the Triple Intervention of Russia, Germany, and France forced the Japanese to return the Liaodong Peninsula in exchange for a greater indemnity from China. To add insult to injury, these three powers then began carving up parts of China amongst themselves in 1898, with Russia gaining a 25-year lease of the Liaodong peninsula, a particular thorn in the side of the Japanese government and public.

Then came the Russo-Japanese War of 1904-1905, when the Japanese once again showed their prowess as a rising power on the global stage. Though the Japanese did sustain far greater casualties than they had in the Sino-Japanese War, their crushing defeat of the Russian Baltic Fleet at the Battle of Tsushima Strait astounded much of the world. The British in particular, were quick to praise Admiral Togo Heihachiro, with some political cartoonists and satirists calling him the "Nelson of the East" (rather amusingly, the Royal Navy also presented him with a literal lock of Nelson's hair in commemoration of his resounding victory). Kenneth Henshall on the effect of the Russo-Japanese War on the Rising Sun's status:

"The war had caused Japan much suffering in human as well as financial terms, but it had earned the nation great respect internationally. Russia was by no means the strongest of the Western powers, but it was the largest, and Japan's victory over it was a rare victory in modern times over a western nation by a non-western nation

Japan had now achieved its aim of matching and being taken seriously by the Western powers. Far from being faced with the threat of colonisation by imperialist powers, as it had feared a few decades earlier, it was now poised to take its place among them.

In the next part of the response, we shall see how Japan's rise to be an "equal" power was swift and irreversibly shattered by the end of the First World War, and how the reign of the Taisho emperor (r. 1912-1926) would mark the rise of right-wing elements in Japan's government and citizenry.

Part 1 of 3