The liberal wing of the Taiwanese anti-colonial movement, which Cai represented, went through three distinct phases, moving from demands for assimilation into the metropole in 1914 to demands for self rule in the 1920s. The failure of the movement to achieve its goals led to its marginalization in the 1930s and the rise to prominence of Marxist activists.
Cai first became involved in the movement as a member of the Assimilation Society (Dôkakai). The Society sought to promote "harmonious relations between Japanese and Formosans based on the concept of racial equality" (Chen 1972, 479, quoting Article 3 of the association's charter in Nihon tôjika no minzoku undô II, 17-18). What this actually meant in practice was ambiguous. On the one hand, Taiwan was to be considered fully a part of Japan and Taiwanese people equal to Japanese people. On the other, Taiwan was also to be treated as a unique place, a special kind of bridge or pathway between Japan and China.
It's possible to see the contours of this ambiguity in the speeches that Itagaki Taisuke (板垣退助), a liberal member of the Japanese parliament, gave as part of the first meeting of the Society. In 1914, Lin Hsien-t'ang (林獻堂), the Society's founder, invited Itagaki, to travel to Taiwan and serve as the society's first president. While in Taiwan, Itagaki gave three speeches, each of which advocated for the "assimilation" (dôka) of Taiwan into the metropole.
On the one hand, Itagaki described assimilation as a process of expanding the cultural and political boundaries of Japan to fully incorporate Taiwan. Itagaki argued for the complete absorption of Taiwan into the political, economic, and cultural space of Japan through "equal economic opportunity, legalization of mixed marriage, propagation of Japanese language, more emigration from Japan, and the publication of a newspaper by the Assimilation Society to promote friendship between the two peoples" (Chen 1972, 480, citing Itagaki Taisuke zenshû, pp. 395-412).
At the same time, Itagaki's vision of assimilation imagined a boundary between Japan and Taiwan, and Taiwan and China, that Taiwanese Chinese elites would be expected to transit as part of their special duties as Japanese subjects. As he talked about absorbing Taiwan into Japan, Itagaki portrayed Taiwan as a "path" between Japan and China. Taiwan would unite the two countries as a bulwark against Western imperialism in Asia.
The purpose of our Assimilation Society is, in addition to achieving the goal of assimilation, is to open a pathway (michi) for friendly relations with China. The enactment of this organization's goals should not be the cause of any trouble for you Taiwanese islanders [Taiwanese Chinese] (Itagaki 1969, 400).
Unfortunately for Cai, the Assimilation Society did not last long. The ethnic Han Chinese population of Taiwan supported Itagaki's message, particularly the more elite families. In the association's first month, 3,198 members signed up. Only 44 were Japanese. Yet the idea of assimilation unsettled the colonial government. The Government General of Taiwan watchfully tolerated Itagaki while he toured Taiwan. One month after his departure, however, the government's tone abruptly changed. The island's newspaper harshly criticized Japanese members of the association, the police revoked the association's permit to meet, and the association was effectively outlawed (Chen 1972, 480). Cai was fired from his job as a teacher at Tainan Number 2 Common School for participating in the society.