Yes, you can play almost all the factions when you complete the grand campain. The only exseptions is the India factions, and the two invading. (mongols and someone else)
complete campaign as the first three roman factions.
In order to unlock the other factions, you must complete campaign for one of the starting factions : Julii, Scipii, and Brutii
for multiplayer you must do it online to unlock them and for campaign it will unlock if you do what is required to unlock the camouflage.
No but there is another way
There is a mod you can download to play as them in the main campaign, you cannot play as the minor nations through unlocking them and their are no patches planned.
complete the campaign then you will unlock it
Playing out campaings.
in data there is not world,why
Playing out campaings.
You don't unlock it just click rewards available
When you beat the campaign you unlock ratchet and shockwave.
Unlock Halo 3:ODST's Campaign Complete Achievement. Once you beat the ODST campaign, visit Halo Waypoint on the dashboard to unlock the item.