This is really three questions rolled into one. I’ll do my best to answer them all although I’m not sure if I can. The first question is “How/Why did Japan modernize in the 19th century?” This is part of my area of expertise. The second question is “Why didn’t Thailand or other Asian Nations achieve the same thing?” I’ll admit that I know very little about Thailand so hopefully some other user can answer that part of the question. I can go over why China and Korea failed to westernize however. The 3rd question is the hardest, and while I’ll take a stab at it, it’s really beyond me. That question is “What conditions prevent or allow a civilization to modernize?”

First, let’s go over the events of the Meiji Restoration. The Meiji government came to power during the Baku Matsu, the fall of the Tokugawa Shogunate. The Bakumatsu was brought on by a combination of growing internal weakness in Japan and foreign pressure. Westerners had been steadily increasing pressure on Japan during the first half of the 18th century. Whaling ships in Japanese waters were starting to become a major problem for the Japanese government which had traditionally considered them a threat and even issued an edict in 1825 ordering Japanese to shoot Westerners and their ships on sight. In the wake of the Opium Wars and the defeat of China, Japan began engaging in limited westernization of its military.

The Shogunate proved incapable of defending Japan however as evidenced by Matthew Perry’s fateful visits in 1853 and 1854. The American squadron of four ships sailed into Edo Bay and forced Japan to accept the first of the “unequal treaties” at gunpoint. Sensing blood in the water, other European powers quickly followed suit forcing Japan to open its borders to trade and allow diplomatic relations to be established (along with other concessions, such as granting extraterritoriality and allowing foreign agencies to collect tariffs) . This was a tremendous loss of face for the Shogunate which based its legitimacy off its ability to defend Japan. Combined with its serious financial problems (due to a number of things, chief among them a lack of currency brought on by the depletion of silver mines), the Shogunate was now in an exceptionally weak position. The deaths of the sitting Shogun and reigning emperor left them even weaker.

Numerous problems that had been festering in Japan came to head at once. The Tozama Daimyo (whose families had sided against Tokugawa 250 years earlier) were upset at their lack of political representation and sided with other Daimyo upset at the Shogunate’s inability to defend Japan from the West. A rebellion in the name of restoring power to the Emperor was started and eventually managed to take control of most of the country. In 1867, recognizing his position was untenable; the last Tokugawa Shogun stepped down and ceded authority to the new government, which we know as the “Meiji government.” To consolidate its power, the Meiji government immediately “encouraged” Daimyo to “spontaneously” surrender their Domains to the emperor. This was achieved by 1871 and the modern prefecture system was born.

The Meiji government adopted the slogan “Enrich the country, Strengthen the Army.” Their goal was to achieve parity with West which they considered necessary for the national security of Japan. Short-term, this meant renegotiating the “unequal” treaties which Japan had been forced to sign. In the longer term it meant creating institutions, legal, political, economic and military, that could provide Japan with the power to counter the West. As part of its policy of reform, the Meiji government worked hard to make sure that foreign problems would not interrupt its domestic reforms. Attacks on foreigners were outlawed and the law was enforced. Japan also refrained from foreign adventure until 1872 when a minor expedition was sent to Taiwan to appease public opinion (which called for Japan to be more assertive). Japan as a country also shifted its focus entirely from China to the West during the early Meiji government. Whereas earlier Japan had seen China as the center of the world, in response to China’s defeats in the Opium Wars and growing Western influence, Japan instead focused on the West. This is evidenced by another Meiji government slogan, “Japanese Spirit and Western Learning.”

In 1871, in order to collect knowledge on Western Practices and attempt to renegotiate the “unequal” treaties, the Japanese sent out an embassy to visit Europe and the United States. The Iwakura Mission as it was known, differed from similar efforts made earlier in that it consisted entirely of high-level government leaders. They had the power to implement changes in Japan’s society when they saw the need for them. The mission was unsuccessful in convincing the Western powers to change the “unequal” treaties, but he men involved returned with the realization that if Japan wanted to compete with the West, it would have to adopt more than Western Technology. Western Institutions and Education were key to Western power as well. Ito Burumi, one of the key players in the Meiji Restoration and a member of the Iwakura Mission said to Americans in Sacramento:

We came to study your strength, that by adopting widely your better ways we may hereafter be stronger ourselves… We shall labor to place Japan on an equal basis in the future with those with those countries whose modern civilization is now our guide”

As a result of this mission, Japan drove to mimic not only Western military strength, but also their economies, education systems and government styles. Over the next 20 years Japan would make massive leaps and bounds towards Westernization. The Samurai were gradually crushed, with their stipends being turned into government bonds by 1874 and the legal basis behind their privileged position in society removed. In 1873 the Meiji government implemented nationwide conscription, creating a modern Army that drew from the entire population rather than just from the Samurai class. A centralized education system which taught Western ideas was established. Government was centralized and the Daimyo who didn’t adapt, deprived of their power. The crowning achievement of the Meiji Restoration was the new constitution adopted in 1890 that created a Prussian style government with an elected diet, a powerful cabinet and an Emperor who served as absolute ruler. Industrialization also began to take off in the 1890s and, with a Western government structure, economy, and military Japan was well on its way to competing with the Western Powers.