Final answer:
Siam, known today as Thailand, was the Southeast Asian country that remained independent during the period of New Imperialism, unlike its neighbors that were colonized by European powers.
Step-by-step explanation:
The country in Southeast Asia that remained independent during the period of New Imperialism was Siam, which is known today as Thailand. While France and Britain colonized most of the mainland region of Southeast Asia, with Britain taking control of Burma and France establishing a stronghold over Indochina (present-day Vietnam, Laos, and Cambodia), Siam was able to navigate the geopolitical complexities of the era to maintain its sovereignty.
Siam skillfully managed its foreign relations, engaging in diplomatic negotiations and modernizations that allowed it to avoid being colonized. For instance, it made territorial concessions to both Britain and France to keep the larger powers at bay. The Japanese occupation during World War II also saw the creation of puppet states in the region, but it was after the war that these countries, alongside Siam, eventually moved towards complete independence and self-governance. Thus, Siam (Thailand) stands out as an exception during a time when imperialism was reshaping the political landscape of Southeast Asia.