Why can't I use my regiment in battle? And what is that purple circle icon on my regiment?
When you see a circling purple portal icon on a regiment's icon, it means that your regiment is currently on an Expedition in the Sky Arch, and thus cannot be deployed in battle.
Don't worry though, as soon as the expedition has been completed you can claim your rewards at the Sky Arch. Your regiment will be ready to fight once again! You can also choose to cancel an ongoing expedition at any time, however, no rewards will be earned in such case.