Thailand, officially the Kingdom of Thailand and formerly known as Siam, is a country in Southeast Asia. Located at the centre of the Indochinese Peninsula, it is composed of 76 provinces spanning 513,120 square kilometres (198,120 sq mi), with a population of over 66 million people; Thailand is the world's 50th-largest country by land area and the 22nd-most-populous. The capital and largest city is Bangkok, a special administrative area. Thailand is bordered to the north by Myanmar and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and the southern extremity of Myanmar. Its maritime boundaries include Vietnam in the Gulf of Thailand to the southeast, and Indonesia and India on the Andaman Sea to the southwest. Nominally, Thailand is a constitutional monarchy and parliamentary democracy; however, in recent history, its government has experienced multiple coups and periods of military dictatorships.
Tai peoples migrated from southwestern China to mainland Southeast Asia from the 11th century; the oldest known mention of their presence in the region by the exonym Siamese dates to the 12th century. Various Indianised kingdoms such as the Mon kingdoms, Khmer Empire and Malay states ruled the region, competing with Thai states such as the Kingdoms of Ngoenyang, Sukhothai, Lan Na and Ayutthaya, which rivalled each other. Documented European contact began in 1511 with a Portuguese diplomatic mission to Ayutthaya, which became a regional power by the end of the 15th century. Ayutthaya reached its peak during cosmopolitan Narai's reign (1656–1688), gradually declining thereafter until being ultimately destroyed in the 1767 Burmese–Siamese War. Taksin (r. 1767–1782) quickly reunified the fragmented territory and established the short-lived Thonburi Kingdom. He was succeeded in 1782 by Buddha Yodfa Chulaloke (r. 1782–1809), the first monarch of the current Chakri dynasty.
Throughout the era of Western imperialism in Asia, Siam remained the only nation in the region to avoid being colonized by foreign powers, although the Siamese government was often forced to cede both territory and trade concessions in unequal treaties. The Siamese system of government was centralized and transformed into modern unitary absolute monarchy in the reign of Chulalongkorn (r. 1868–1910). Siam joined World War I siding with the allies, a political decision to amend the unequal treaties. Following a bloodless revolution in 1932, Siam became a constitutional monarchy and changed its official name to "Thailand". Thailand was a satellite of Japan in World War II. In the late 1950s, a military coup under Field Marshal Sarit Thanarat revived the monarchy's historically influential role in politics. Thailand became a major ally of the United States, and played a key anti-communist role in the region as a member of the Southeast Asia Treaty Organization (SEATO). Apart from a brief period of parliamentary democracy in the mid-1970s, Thailand has periodically alternated between democracy and military rule. Since the 2000s, Thailand has been caught in a bitter political conflict between supporters and opponents of Thaksin Shinawatra, which culminated in two coups, most recently in 2014 and the establishment of its current and 20th constitution and faces the ongoing 2020 Thai protests.
Thailand is a founding member of the Association of Southeast Asian Nations (ASEAN). Despite comparatively sporadic changes in leadership, it is considered a middle power in global affairs. With a high level of human development, the second-largest economy in Southeast Asia, and the 20th-largest in the world by PPP, Thailand is classified as a newly industrialized economy; manufacturing, agriculture, and tourism are leading sectors of the economy.
Vietnam (also written as Viet Nam, Vietnamese: Việt Nam, [vîət nāːm] (listen)), officially the Socialist Republic of Vietnam (Vietnamese: Cộng hòa Xã hội chủ nghĩa Việt Nam), is a country in Southeast Asia and the easternmost country on the Indochinese Peninsula. With an estimated 97.8 million inhabitants as of 2020, it is the 16th most populous country in the world. Vietnam shares its land borders with China to the north, and Laos and Cambodia to the west. It shares its maritime borders with Thailand through the Gulf of Thailand, and the Philippines, Indonesia and Malaysia through the South China Sea. Its capital city is Hanoi, and its most populous city is Ho Chi Minh City.
Archaeological excavations indicate that Vietnam was inhabited as early as the Paleolithic age. The ancient Vietnamese nation, which was centered on the Red River valley and nearby coastal areas, was annexed by the Han dynasty in the 2nd century BC, which subsequently made Vietnam a division of Imperial China for over a millennium. The first independent monarchy emerged in the 10th century AD. This paved the way for successive imperial dynasties as the nation expanded southward until the Indochina Peninsula was colonised by the French in the late 19th century. Modern Vietnam was born upon the Proclamation of Independence from France in 1945. Following Vietnamese victory against the French in the First Indochina War, which ended in 1954, the nation was divided into two rival states: communist North and anti-communist South. Conflicts intensified in the Vietnam War, which saw extensive US intervention in support of South Vietnam and Soviet and Chinese support for the North, and ended with North Vietnamese victory in 1975.
After North and South Vietnam were reunified as a communist state under a unitary socialist government in 1976, the country became economically and politically isolated until 1986, when the Communist Party initiated a series of economic and political reforms that facilitated Vietnamese integration into world politics and the global economy. As a result of the successful reforms, Vietnam has enjoyed a high GDP growth rate, consistently ranked among the fastest-growing countries in the world. It nevertheless faces challenges including corruption, pollution, poverty, inadequate social welfare and a poor human rights record, including increasing persecution of religious groups and human rights advocates and intensifying restrictions on civil liberties. By 2010, Vietnam had established diplomatic relations with 178 countries. It is a member of such international organisations as the United Nations (UN), the Association of Southeast Asian Nations (ASEAN), the Asia-Pacific Economic Cooperation (APEC) forum, and the World Trade Organization (WTO).