More importantly the US wasn’t waging “war” against the “North”. They were waging a genocidal destruction campaign against all Vietnamese, mainly in the US controlled South as a means to keep the region destabilized and prevent it from emerging as an economic competitor in the sphere of UDSSR/China.

So if you were Vietnamese in the North or South, Rice farmer or of another profession, chance was US being out to kill or subdue you, so resistance was the only option.