Mongols decisively defeat Vietnam

Okay so in 1258 the Mongols invaded Vietnam-they thrashed the Vietnamese badly but the Vietnamese retained nominal indepedence.

What would it take for Vietnam to be made into a full province?
 
Nothing. Maybe successfully capture the royal family or lure the Vietnamese army out into the open to destroy them or successfully invade Champa and surround Dai Viet. Even then it would be extremely difficult, almost neigh impossible, to stamp out resistance.

The Mongols may declare Vietnam to be de jure a province of the empire but officially the region will never be pacified.

Expect guerrilla warfare for decades to come even after the Mongols took Thang Long and a successful rebellion to happen the moment the Yuan dynasty inevitably fall.
 
Send 500k troops into our land, support with naval (more like river and close-shore boats though) and high food storage... Oh, right. They did it in 1284/85, and still get humiliated.

Though for 1258, they can really send more troops (only 50k in OTL, I think), less Calvary and more logistics. Also try to kill a lot of Vietnamese, it helps pacifying the land.
 
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