The US military did have and used the plans to invade Afghanistan drawn up prior to 911. It’s not so nefarious as you might think however cause the US military also has plans to invade Canada, Mexico, and many other countries for which an invasion would be unthinkable. These are not hypothetical plans either but very detailed plans. The US military even has plans for a zombie 🧟♀️ apocalypse. Literally!
Why? Two reasons, Because good planners require practice. Secondarily you never know what the future will bring. The plans to invade Afghanistan where drawn up in an age when the Soviet Union had invaded and got bogged there. It would have been the last place an American administration would seek adventure. Yet after 911 the plan went into effect. Extraordinarily!
The plan incidentally was to use minimal military footprint and rather to send representatives with money to buy support. During the Bush administration which executed the invasion there were fewer American soldiers in Afghanistan than police officers in Manhattan island. Good plan.
The pentagon defends spending money on a zombie invasion plan by suggesting that components of the plan could be used for more realistic threats. As well as the training benefits.