For over a century the longbow reigned as undisputed king of medieval European missile weapons. Yet only England used the longbow as a mainstay in its military arsenal; France and Scotland clung to the technologically inferior crossbow. This longbow puzzle has perplexed historians for decades. We resolve it by developing a theory of institutionally constrained technology adoption. Unlike the crossbow, the longbow was cheap and easy to make and required rulers who adopted the weapon to train large numbers of citizens in its use. These features enabled usurping nobles whose rulers adopted the longbow to potentially organize effective rebellions against them. Rulers choosing between missile technologies thus confronted a trade-off with respect to internal and external security. England alone in late medieval Europe was sufficiently politically stable to allow its rulers the first-best technology option. In France and Scotland political instability prevailed, constraining rulers in these nations to the crossbow.