Why did we Invade Afghanistan?

The Taliban had banned the production of poppies or Papaver somniferum; we did not like this because we need our morphine and codeine.  Since we invaded Afghanistan the production went back up and in 2008 Afghani farmers were licensed to grow and sell their poppy crops to pharmaceutical companies.  http://en.wikipedia.org/wiki/Opium

Can anyone give a better reason to be there, to kill and be killed,  than to protect production of our needed Morphine and codeine?