Allies in Arms has your Rooks start every fight you order with a volley, softening up Blighters before the melee starts. It is a Ringleader upgrade.
Allies in Arms is entry 2 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £2,500, 50 Metal, 25 Chemicals. Before Allies in Arms unlocks you need Rook Training I.