Three Little Reasons

France wanted to invade Vietnam because of a few reasons:

1.  French wanted to protect the missionaries that were being attacked by the Vietnamese government. Tu Dac, the Vietnamese emperor at the time, attempted to eliminate all Christians from the mainly Buddhist country. [D] 
2. During a time of imperialism, the French did not want to be left behind in the race of global power. Everyone wanted a piece of foreign territory for raw materials and new markets. Vietnam was a direct trade center with China and would expand territory for French capitalism. As Napoleon’s specially appointed committee (to provide rationale behind the invasion) pointed out: “will we be the only ones without possessions in Asia, where the English, Dutch, Spanish, and even Russians are strengthening their positions?” [D] Napoleon also had an “obsession with national glory” [D]. Since England was establishing herself in Asia, why shouldn't France as well? 
3. France was also interested in Vietnam for its trade with China. With raw materials of coffee, rubber, and tea, Vietnam was a place for imports and exports of goods. [E]

All of these reasons to Napoleon's invasion. Forward march the French army! Huzzah! 

The First Conquest

Napoleon III, who had heard of good things of the Vietnamese area from the troops stationed there to protect the Catholic missionaries, started the initial conquest of the Vietnamese in July 1857. Rigault de Genouilly, a naval commander who advocated military action to control Vietnam, arrived at Tourane (today Da Nang) in August 1858 with 14 ships and 3,500 troops to turn the port city into a military base. They attacked on September 1^st, occupying the town a day later. The army then sailed to Saigon in February 1859 and seized the city two weeks later. By 1861, the French had occupied three adjacent provinces that Tu Dac needed to give up because the French armies were starving the country [D]. Finally, in June 1862, the French signed a peace treaty that gave them new territories, the entire colony dubbed ‘Cochinchina.’ In August 1883, the French colony expanded once more, so that by 1893, the Indochinese region was established, consisting of the Cochinchina area, Annam, Tonkin, Cambodia, and Laos. By using brute force and advanced technologies, the French had colonized Vietnam [C].

Splitting up the pie: Indochina Region ruled by the French in 1893

Timeline [C]:

Date	Event
July 1857	Napoleon III ordered the colonization of Vietnam
August 1858	Rigault de Genouilly sailed to Tourane and occupied the city
February 1859	French forces occupied Saigon
1861	French occupied three adjacent provinces to Saigon that Tu Dac resigned over
June 1862	Peace treaty signed by French and Vietnamese, entitling French to the “Cochinchina” region
1893	Indochinese area established