Washington considered the nuclear option to save its ally. That option later fell, and with it French influence in Indochina.
The French Minister was surprised by the generous American offer, and did not take it seriously. Neither France nor its forces were prepared for a nuclear scenario. The United States practically financing about 78% of the cost of the Indochine War. The Pentagon Papers published in 1971 were actually military and logistical aid to France for fear of communist expansion and China’s control of the region.