Under what conditions do coalition partners fighting alongside each other in a war take the next step and field their forces together in battle? While states frequently go to war with partners, they do not always fight side-by-side on the battlefield. Existing scholarship focuses largely on the motives of coalition partners to deepen their cooperation when considering why such actors fight battles together. We argue that, for coalition partners to employ their forces side-by-side in combat, they require the opportunity as well as the motive to do so. More specifically, we contend that coalition partners’ logistical capacities—their abilities to transport and sustain troops and associated equipment to the battlefield—exert a powerful influence on the creation of battlefield coalitions.