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A general trend I have noticed in French history, dating from the fall of France after the defeat at Waterloo and the subsequent losses in the Congress of Vienna, is that there was and is a strong urge to regain a sense of international prestige. Throughout the nineteenth century France slowly degraded from being the world capital of liberal democracy and intellectualism. In the latter half of that century, France suffered an embarrassing and crushing defeat by the newly formed power of Germany in the Franco-Prussian war. This defeat was then compounded by the utter devastation of what was once the most feared military force in the world.
Even in much of the 20th century France is seen as a major world power. As a personal aside, my mother's generation in the Netherlands learned French as their first foreign language, and despite the rise of America, it wasn't until roughly generation X that English replaced French in this regard. Taken all together this explains much of France's attempts to maintain their cultural identity and obtain positions of international importance (such as their attempt to be the home of the EU government among other things). While there are some economic arguments that can be made on the behalf of French colonialism to justify French action in Indo-China, they seem to be much less important than the irrational cultural motivation for doing so (similar to America's need to invade overcome the "Vietnam syndrome" to 'justify' the overblown invasion of Grenada).
Upvote:-1
There's a lot of good sense, and even the occasional stray fact, in the answers above, but there is one slight error of perspective winding through them all: all treat "England" and "France" as single forces with single policies and interests. This is unreal.
In the period 1945 to 1950, both countries were strongly divided on left-right lines. The Communists came very close to taking power fairly, by honestly winning an election, in France. Labour took power in Britain in 1945.
In both countries, the left was foundationally anti-imperialist, anti-colonialist, and contemptuous of their Tories (England) and failed militarists, France. The Labour government withdrew from India and Pakistan, and it was a Socialist President, Mendes-France, who surrendered after Dien Bien Phu.
Upvote:13
There is something to BrotherJack's answer but just considering it an “irrational” urge for greatness and prestige is a little short-sighted.
A few other factors:
France had a huge colonial empire, not only Indochina. Defending its claim to Indochina was also a way to show it did not intend to give up all this. It's still debatable whether colonies in general are a net gain for the mainland but it's clearly a much larger issue. In fact, in the immediate post-WWII period, British – not French – troops briefly assumed control of the region after the surrender of the Japanese, because Britain also had a stake in making sure the old world order would not be upset.
It's only after the Indochina war that it became clear that old-style colonialism was unsustainable in this day and age and that France decided it was just as beneficial to have ostensibly independent states under its influence rather than actual colonies. It then initiated/encouraged the independence of its African colonies (with the - huge - exception of Algeria).
And for all the setbacks and ugliness of it all (up to the attitude France took during the genocide in Rwanda), France is still very influential in many parts of its former colonial empire so you cannot easily discount this policy as wholly irrational and emotion-driven. It does serve some interests really well.
France did not immediately commit huge resources and probably did not realise what it was up against. Just like the US later, it offered support to a local surrogate state and then got drawn in a much broader conflict than expected, ramping up forces along the way (many of the soldiers came from the rest of the French colonial empire by the way).
Nobody thought “let's sacrifice 100 000 men to the greatness of France“ (that's excluding local forces), it was more like “let's send 20 000 soldiers and we will quell the revolt like we did before”. You have to realise that uprisings and violent protests were relatively common, France never had a very deep control over its colonies.
It was a cold war conflict. France fought against the Viet Minh guerrilla between 1945 and 1949 but the war changed completely in 1950 (also the time of the Korea war, incidentally). After that, France fought an open war against a formidable Chinese-backed communist force, not only because of narrow interests but for broader geopolitical reasons.
I have read somewhere on this site that the Vietnam war was the US somehow stepping up to rescue France from its incompetence but that's nonsense. It's exactly the other way around: France fought for much longer than it should (and possibly would otherwise) have at the behest of and with logistical and financial support from the US as part of the global fight against communism.
Finally, and I am not sure this is a major factor in this particular war, but it's interesting to know that colonialism wasn't simply a top-down process, neatly planned and engineered from the centre. You can see that when you look at how Indochina was conquered in the first place. Local French forces gradually took control of the territory (often ostensibly reacting to some threat or slight from a local leader) and then asked for more support from the mainland. The colonial elite also had strong lobbies to represent their interests back home.
Historical research also shows that most people in mainland France simply did not care much about the Indochina war, political parties (even the French communist party!) did not make it an issue. It's not like the whole of France united behind it or fought bitterly over it (as happened during the Algerian war).