Indochina is separated from China by a heavily jungled, mountainous region of incredible ruggedness and striking beauty, not readily penetrated by commerce; this was to form an effective barrier to southward Chinese population movement, though not to cultural and political influence. The main points of entry from the north are through a handful of mountain passes and the Red River valley. Geographically, eastern Indochina is dominated by the Annamite mountain chain, the Red River and the Mekong River, all running generally northwest to southeast. The Annamite chain separates the valleys of the Red and the Mekong rivers and follows the modern Vietnamese/Laotian border to a point some fifty miles north of Saigon with heavily forested peaks ranging in height from 5,000 feet to 8,500 feet; the chain is penetrated by a limited number of east/west passes, notably Barthelemy pass, linking the coastal plain to the Plaine des Jarres in Laos, and Nape and Mhu Gia passes linking the coastal plain to the middle Mekong.
The Red River rises in southwestern Yunnan and flows southeast, broadening into a delta which forms the demographic and political heart of northern Vietnam. The Mekong originates in the Himalayas, flows south through Young into northern Laos, and delineates the present Thai/Laotian boundary before running back into Laos; it then bisects Cambodia before entering Vietnam southeast of Saigon and spreading out into a broad delta. By the logic of culture and geography, the middle Mekong should be the heart of a Lao nation encompassing most of what is today northwestern Thailand, and so indeed it was from the mid-fifteenth century to the late seventeenth. But civil war weakened the country and Laos was in the process of absorption by Thailand when French annexation in 1893 fixed the border at the Mekong. Similarly, the lower Mekong and its delta should be the heart of the Khymer nation an once were, but, as with Laos, French annexation in 1863prevented the absorption of Cambodia by its neighbors. There was, however, an important difference: while the Thai and the Lao are closely related, with mutually intelligible languages, the Khymer are culturally and linguistic ally distinct from the other peoples of Indochina. In the ninth and tenth centuries, Khymer kings ruled most of Indochina west of the Animates from their capital at Angkor Wat, but the kingdom contracted under Thai and Vietnamese pressure and was in decline when Europeans reached Southeast Asia. But for French intervention, the Khmer would likely have been reduced to a dwindling population under Vietnamese and Thai rule; indeed, the Mekong Delta, conquered by Vietnam in the late 1700s, still has a Khymer minority as do the border provinces of southeastern Thailand. The Mekong Delta, a highly fertile rice-growing region, dominates southeastern Indochina physically and economically much as the Red River Delta dominates the north.
Geography did much to shape the history of Vietnam. The Annamite chain channeled Vietnamese expansion, concentrating settlement in the Red River Delta and the narrow coastal plain; it also channeled the flow of ideas, determining that the main intellectual influence on Vietnam would be Chinese rather than Indian and that Bhuddism and Confucian forms of government would prevail. Geography and culture determined that the Vietnamese would occupy the small portion of the land suitable for paddy rice cultivation, outnumbering the aboriginal inhabitants and pushing them back into the highlands. The monsoon, the seasonal wind which flows steadily from the northwest in winter, then reverses during the summer, reinforced the influence of geography. During the winter, moist winds sweeping off the South China Sea and the Gulf of Tonkin back up against the mountains and turn to rain, providing the rice cultivators of the Red River Delta and the coastal plain with a reliable source of water for irrigation. The summer monsoon brings rain to the west of the mountains and clear weather to the east. The predictability of the monsoon and its rains is central to the Vietnamese way of life and to the military history of Vietnam, for military forces move with difficulty, if at all, during the wet monsoon.
Page 3 (America in Vietnam)