the process of vietnam's colonisation began in september 1858 when a franco-spanish force landed at da nang in central vietnam and attempted to proceed to the capital, huế. after becoming tied down, they sailed to the less-defended south. the french and spanish quickly captured the imperial citadel of saigon in february 1859—the fortress's commander committed suicide. the citadel was razed and the substantial supplies were confiscated. the leaderless and defeated imperial troops fled in disarray. the attacks were ordered by french emperor napoleon iii. french diplomats, naval officers, merchants and missionaries had long advocated the expedition. the missionaries wanted the french administration to facilitate their work converting the vietnamese to roman catholicism, while military and business figures saw commercial opportunities in vietnam. napoleon's motivations were primarily imperial, strategic and commercial, but he found it convenient to cite "freedom of religion" as his justification for taking action. the nguyễn dynasty was confucianist and had restricted the activity of missionaries. the belief system of christianity was incompatible with the confucian belief that the monarch was the "son of heaven".