One has to think twice about changing history:
I've seen some posts in here about preventing Pearl Harbour Dec 7,1941.
-It is very likely Hitler would've eventually taken Britain without Normandy Invasion and Invasion of Sicily. Both were led by US forces.
Also the Japanese would have attacked other areas and expanded their empire across Western Pacific.
-Preventing 9/11 would mean continued suffering and hardship of women under Taliban rule in Afghanistan.