At least legally, the US had no “rights” over Afghanistan. Afghanistan was not a colony, territory or state of the US. The government was “free” to kick out the US whenever they wanted.

The president was a US-educated pawn. No chance he would've kicked out the US, which is why the US put him there.

That didn't afford the US any right to annexation.

I agree 100% in principle.

In practice, US foreign policy doesn't recognize international law and never really has.

The US could annex Afghanistan, and apart from the CCP doing a lot of complaining about it (primarily because they want access to Afghanistan's mineral wealth), nothing would happen.