Understanding the culture and the people of the enemy
Wealth to fund the war
For an imperialist power seeking to successfully subdue and occupy a nation: willingness and ability to kill and destroy the enemy at genocidal levels. (It worked in Germany and Japan. We haven't killed at that level since and also have not succeeded in successfully subduing and occupying anyone except Panama and Grenada.
You can dance without a lawyer
1) The American Revolution wasn't a guerilla war.
2) The Mujahadeen didn't begin seeing success, until they were supplied with conventional weapons and training in conventional tactics.
3) As I pointed out, The Viet Cong was destroyed in 1968.
4) The Communists fighting against The Rhodesians won, but through attrition, not tactics. The free world turned its back on Rhodesia, while the commies had material flowing freely from The Soviets, Cuba and East Germany.
Knowing all these details is very important. It damages your credibility when you make an erroneous claim that The Viet Cong was still in existence in 1976.
Scorched earth until your enemy surrenders unconditionally.
Yes it really is that simple.