The main difference is granularity. Terraform runs a plan and applies it as a batch. If something fails, you re-run apply and it retries from the last saved state... but that state is per-resource, not per-API-call.
Alien tracks state at the individual API call level. A single resource creation might involve 5-10 API calls (create IAM role -> attach policy -> create function -> configure triggers -> set up DNS...). If it fails at step 7, it resumes from step 7. Terraform would retry the entire resource.
The other difference is that Alien runs continuously, not as a one-shot apply. It's a long-running control plane that watches the environment, detects drift, and reconciles. Terraform assumes you run it, it converges, and then nothing changes until you run it again.
