Final answer:
Myanmar, also known as Burma, is the Southeast Asian country most threatened by internal tensions, particularly due to ethnic divisions, a history of military rule, and humanitarian crises like the treatment of Rohingya Muslims. Thailand and Indonesia also face internal tensions, but Myanmar's challenges are more substantial and complicated by its relations with neighboring China.
Step-by-step explanation:
The country in Southeast Asia most threatened by internal tensions appears to be Myanmar, also known as Burma. The country has long been dealing with internal conflicts, particularly with ethnic divisions and a history of military rule. Violence against Rohingya Muslims in Myanmar's Rakhine state has led to severe humanitarian crises and has been internationally condemned. This plight of the Rohingya has led to a mass exodus to neighboring countries, creating tensions both within and between nations in the region.
Additionally, Myanmar's economic struggles, coupled with its position between the influential powers of India and China, further complicate its internal stability. With China playing a significant role in supporting the Myanmar military regime and involvement in infrastructure projects, Myanmar faces the challenge of navigating between appeasement of its economically important neighbor and the need to address its domestic issues.
Thailand also faces tensions, especially between the Buddhist majority government and Muslim minority in the south. However, these do not seem to be on the scale of the conflicts in Myanmar. Indonesia has its own social tensions, such as the treatment of its Chinese minority and conflicts in regions like Papua, but it has maintained a remarkable degree of stability and national cohesion despite these challenges.