And yet democracy had often been established in countries with no tradition of it.
We know today that the US military had partly stabilized Iraq.
Moreover, the United States' conflict in Afghanistan cannot be compared with that in Vietnam in terms of scale. The North Vietnamese had tanks, fighter jets, AA equipment, and murdered over 130,000 of their countrymen, not counting 266,000 ARVN deaths. US casualties consisted of 60,000 men , rounded up. Post-war, the North Vietnamese proceeded to murder 643,000 men.
Further, 1,100,000 Viet Cong and NVA men have been killed in the fighting, as well as up to 270,000 Vietnamese civilians killed by US air strikes, including NV civilians.
This was a conflict on an entirely different SCALE from Afghanistan.