remind me why we are in Afghanistan

The U.S. is going to add approximately 4,000 American troops to its forces in Afghanistan -- the 'war' yet to be declared by Congress after 16 years.

WASHINGTON (AP) -- The Pentagon will send almost 4,000 additional American forces to Afghanistan, a Trump administration official said Thursday, hoping to break a stalemate in a war that has now passed to a third U.S. commander in chief. The deployment will be the largest of American manpower under Donald Trump's young presidency.

Lurking somewhere in the background is some unprovable-but-inflammatory link to the 2001 demolition of the World Trade Towers (et al) in 2001 that stands as the reason for U.S. involvement. But it occurred to me today that I really don't know why -- even from the exceptionalist point of view -- we are there. Is it oil? Is it military strategy? Is it a diversion from problems at home -- the usual legerdemain kind of thing where you are encouraged to look there so you don't see what's here?

Today's news comes on the heels of President Donald Trump's handing the authority to raise troop levels to Defense Secretary Jim Mattis ... a move that, dare it be said, takes the onus off the president when the new infusion of American blood works no better than previous infusions.

Seriously, is there some reasoning, however bereft, for being there?