Trump is looking at Afghanistan as America First.  He has been told of the mineral riches that are to be found there.  His corporate supporters want to get their hands on those riches.  This reason is being carefully hidden, but it has been mentioned in many places.

Though it has been known for many years, the Bush administration had enough wisdom nit to mention it directly. They unlike the current president knew that the idea reeks of colonialism.  It is also an excellent recruit tools for ISIS, Al Queda, etc.

When I was in high school being taught by the Christian Brothers, our history teacher mentioned that when one is looking for reasons for conflict, follow the money.  The money in Afghanistan is in the ground and without a strong military presence in Afghanistan, it will not be safe or feasible to get the minerals out.