As a map some days ago showed, Canada, Greenland, Panama, probably Israel, maybe even Ukrain - it’s all about shipping lanes.
All that effort makes sense if there is the believe that global trade will increase and that the USA will still be relevant when the center of the global population is in Asia.
So extinction is not the goal but could be an accepted risk.
Life will find a way. Going back can be used to create nuclear winter but even then, life will go on.
To completely screw over the world, I would go into the future and acquire the source code for a general AI.
Then the world can be locked down and robots can be built to abuse every human and sentinent being individually, for a very long time.