Legitimacy is not an all-or-nothing construct; a Government is not 100% "legitimate" or "illegitimate". There's little doubt that Ho's successful expulsion of the French endowed him and his movement with a substantial perception of legitimacy. That perception was not universal, especially among those who had a personal vested interest in maintaining the dwindling perception of their own legitimacy, but it was sufficient to attract support and sustain his movement until those who opposed him saw their own perceived legitimacy dwindle (largely through their own actions) to an unsustainable level.
Certainly the capacity to apply force matters, but that capacity, as well as the ability to sustain that capacity through foreign and local support, depends largely on how the balance of perceived legitimacy shifts. That was true in Vietnam and it's true in Afghanistan or Syria.
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