User blog comment:DiegoAnime3000/Just wanted to point this out./@comment-96.254.157.84-20140801065529/@comment-96.254.157.84-20140801070758

Ozpin believes Ironwood is acting too rashly by bringing an army, and he's partly right. By strengthening their numbers, they are baasically inviting a war with the White Fang. Once the Faunus see the humans growing in power, they will start gathering arms themselves, hence the giant robot in the trailer.

And in the case of 'keeping friends close and enemies closer', Ozpin is sensible to keep Cinder with him in Beacon because he knows Cinder won't move any sudden moves while under his eye. Monty confirms this somewhat as Cinder is constantly cautious around Ozpin.