In an analytical review of alliance research, James Morrow posed the title question, “Alliances: why write them down?”[1] A decade and a half later, Keren Yarhi-Milo, Alexander Lanoszka, and Zack Cooper revisit this issue, posing their own title question: “To arm or to ally?” Yarhi-Milo, Lanoszka, and Cooper pose this question through the structural lens…