Daianvan, I fear you are conflating Iraq and Afghanistan. Nato involvement in Afghanistan is at the request of the Afghan government, and - in my opinion - is wholly justified.
The troops are involved in frequent and dangerous contacts with the Taleban, and are trying to protect the country's infrastructure from a bunch of fundamentalists who want to turn the clock back to the Middle Ages.
For a flavour of what they're up against and how they're coping, there's a very good despatch here.