Main article: Nguyen Dynasty

Flag of Colonial AnnamThe West's involvement in Vietnam dates back to 166 BC with the arrival of merchants from the Roman Empire, 1292 with the visit of Marco Polo, and the early 1500s with the arrival of Portuguese and other European traders and missionaries.  Alexandre de Rhodes, a French Jesuit priest, improved on earlier work by Portuguese missionaries and developed the Vietnamese romanized alphabet Quốc Ngữ in Dictionarium Annamiticum Lusitanam et Latinum in 1651.

After Nguyen Anh established the Nguyen Dynasty in 1802, he tolerated Catholicism and employed some Europeans in his court as advisors. However, he and his successors were conservative Confucians who resisted Westernization. The next Nguyen emperors, Ming Mang, Thieu Tri, and Tu Đuc brutally suppressed Catholicism and pursued a 'closed door' policy, perceiving the Westerners as a threat. Tens of thousands of Vietnamese and foreign-born Christians were persecuted and trade with the West slowed during this period. These acts were soon being used as excuses for France to invade Vietnam. Actually, the early Nguyen Dynasty accomplished almost everything the previous great Vietnamese dynasties did (like building roads, digging canals, issuing a legal code, holding examinations, sponsoring care facilities for the sick, compiling maps and history books, exerting influence over Cambodia and Laos, etc), except those feats were not enough in the new age of science, technology, industrialization, and international trade and politics. The Nguyen Dynasty is usually blamed for failing to modernize the country in time to prevent French colonization in the late 19th century.

Under the orders of Napoleon III of France, French gunships under Rigault de Genouilly attacked the port of Đa Nang in 1858, causing significant damages, yet failed to gain any foothold. De Genouilly decided to sail south and captured the poorly defended city of Gia Đinh (present-day Saigon). From 1859 to 1867, French troops expanded their control over all 6 provinces on the Mekong delta and formed a French Colony known as Cochin China. A few years later, French troops landed in northern Vietnam (which they called Tonkin) and captured Hà Nội twice in 1873 and 1882. The French managed to keep their grip on Tonkin although, twice, their top commanders, Francis Garnier and Henri Riviere were ambushed and killed. France assumed control over the whole of Vietnam after the Franco-Chinese War (1884-1885). French Indochina was formed in October 1887 from Annam (Trung Ky, central Vietnam), Tonkin (Bac Ky, northern Vietnam), Cochin China (Nam Ky, southern Vietnam, and Cambodia, with Laos added in 1893. Within French Indochina, Cochin China had the status of a French Colony, Annam was a Protectorate where the Nguyen Dynasty still ruled in name, and Tonkin had a French Governor yet local governments were run by Vietnamese officials.

After Gia Định fell to French troops, many Vietnamese resistance movements broke out in occupied areas, some led by former court officers, such as Truong Đinh, some by peasants, such as Nguyen Trung Truc, who sunk the French gunship L'Esperance using guerilla tactics. In the north, most movements were led by former court officers and lasted quite long, with Phan Đình Phùng until 1895 and Hoang Hoa Tham until 1911. Even the teenage Nguyen Emperor Ham Nghi left the Imperial Palace of Hue in 1885 and started the Can Vuong, or "Save the King", movement, trying to rally the people to resist the French. He was captured in 1888 and exiled to French Algeria. Decades later, 2 more Nguyen kings, Thanh Thai and Duy Tan were also exiled to Africa for having anti-French tendencies.

In the early 20th century, Vietnamese patriots realized that they could not defeat France without modernization. Also, having been exposed to Western philosophy, they aimed to establish a republic upon independence, departing from the royalist sentiments of the Can Vuong movements. Japan served as a perfect example that modernization could help an Asian country to defeat a powerful European empire (Russia - see Russo-Japanese War).

Marxism was also introduced into Vietnam with the emergence of three separate Communist parties (Indochinese Communist Party, Annamese Communist Party, Indochinese Communist Union) and later a Trotskyist movement led by Ta Thu Thau. The Comintern sent Nguyen Ai Quoc to coordinate the unification of the parties into the Vietnamese Communist Party in 1930, in Hongkong, with Tran Phu as the first Secretary General. Later, the party changed its name to Indochinese Communist Party as Comintern, under Stalin, did not favor nationalistic sentiments. Nguyen Ai Quoc was a leftist revolutionary living in France since 1911. He participated in founding the French Communist Party and in 1924 traveled to the Soviet Union to join the Communist International (Comintern). Through the late 1920s, he acted as a Comintern agent to help build Communist movements in Southeast Asia. During the 1930s, the Vietnamese Communist Party was nearly wiped out under French suppression with the execution of top leaders such as Tran Phu, Le Hong Phong, and Nguyen Van Cu.

In 1940, during World War II, Japan invaded Indochina yet kept the Vichy French colonial administration in place as a Japanese puppet. In 1941 Hồ Chí Minh, formerly known as Nguyen Ai Quoc, arrived in northern Vietnam to form Viet Minh Front (short for Việt Nam Độc Lập Dong Minh Hoi). Viet Minh Front was supposed to be an umbrella group for all parties fighting for Vietnam's independence, yet it was dominated by the Communist Party. Within Vietnam, Việt Minh had a very modest armed force, which worked with the American OSS to collect intelligence on the Japanese. From China, other non-Communist Vietnamese parties also joined Việt Minh and established armed forces with backing from the Guomingtang.