Sorry for that gap in posts. Between traveling, applying for a new job, and trying to get a bunch of new Marines painted for an upcoming tournament, I just haven't written much. I'm trying to catch back up, so here's my thoughts on the Ultramarines Chapter Tactics.
The Ultramarines are the archetypical Space Marines, and most Chapters follow their tactics. The Ultramarines do not specialize in any specific form of warfare, but instead are flexible enough to deal with any situation. As such, they have three different combat doctrines that each last a single game turn. They can use each one once a game, and can only use one per turn.
The Tactical Doctrine allows all models in the detachment to re-roll 1’s to hit when shooting. Even better, Tactical Marines get to re-roll all their misses when shooting.
The Assault Doctrine allows your units to re-roll failed charges, with Fast Attack squads gaining Fleet.
The Devastator Doctrine allows all your models to re-roll missed Snap Shots, and makes your Devastator squads Relentless as long as they didn’t disembark from a transport.
The Doctrines mean that your army won’t be as good at any one thing as an army running one of the other Chapter Tactics. However, for a couple turns, you’ll be better than any of them. The Tactical doctrine will greatly improve your firepower for a turn, and makes Tactical squads actually threatening. The Assault doctrine will be less useful for most armies, but they will help you if you have any really important charges that you just have to make. Finally, the Devastator doctrine can help shoot down flyers or defend against assaults using Overwatch. If you’ve got Devastators, they can also use the turn to redeploy. Note that it does work if the Devastators stay in their transport, so they can fire out of their Rhino if it moves 6”.
The Ultramarines Combat Tactics encourage a flexible, well-rounded army. They get great bonuses for Tactical squads, assault units, and Devastator squads but only for one turn each. An army including all three types of unit will get the most benefit from the combat doctrines, and judicious use of the doctrines will provide a tremendous boost to your army when you need it.