When you sign in with iCloud, your Iron Marine's Invasion progress will be saved automatically. This means that you'll be able to pick up where you left off, even if you uninstall the game or switch phones, as long as you use the same account and have a connection to the cloud.
Before you start
Make sure you have Game Center updated.
- Check for an update going to Settings > General > Software Update
Check that Cloud Save is enabled
Open your game, go to the Settings menu, and make sure iCloud is on.