Whoa brittle first-strike or burst gang the Ultramarines out-cycle (any fast, front-loaded faction; the phalanx natural prey and its most dangerous foe at once)
Termscampaign-level feud: while it stands, the Ultramarines gain bonus reputation for a win against the quarry but the quarry hunts back for the fast kill; the feud runs until one side is driven off the contested world
The natural foe of the phalanx is the fast, front-loaded gang built to break a line before its discipline compounds — a brittle alpha-strike the Ultramarines are drilled to out-cycle, and the most dangerous enemy they can face. Such a force becomes a standing feud. While it holds, the Ultramarines gain bonus reputation for any battle won against the quarry, but the quarry hunts back hard, pressing for the fast kill before the doctrine ladder can spin up. The feud runs until one side is driven off the contested world, a duel of tempo against tempo that only ends when a whole warzone is decided.
The Disciplined Alliesalliance
Whothe Codex-disciplined Imperial forces of the sector (a loyal Guard gunline, a like-minded loyal Chapter, or any drilled Imperial detachment that fights by the book)
Termscampaign-level alliance: while it stands, the Ultramarines and their ally observe a map-level non-aggression and may lend one favour to each other per post-battle sequence; both gain a small reputation bonus for a campaign won together
The Ultramarines keep good company. A loyal Guard gunline, a like-minded Chapter, any drilled Imperial detachment that fights by the book — the strike force will stand beside forces that share its respect for order, and the pact runs on that shared discipline rather than on any oath sworn in blood. While the alliance holds, neither force contests the same ground against the other. Each may lend the other a single favour per post-battle sequence. Carry a campaign to victory together and both earn a measure of reputation, for a war won in good order honours everyone who kept the line.