From the Baltic to the South China Sea, newly assertive authoritarian states sense an opportunity to resurrect old empires or build new ones at America's expense. Hoping that U.S. decline is real, nations such as Russia, Iran, and China are testing Washington's resolve by targeting vulnerable allies at the frontiers of American power. The Unquiet Frontier explains why the United States needs a new grand strategy that uses strong frontier alliance networks to raise the costs of military aggression in the new century. Jakub Grygiel and Wess Mitchell describe the aggressive methods rival nations are using to test U.S. power in strategically critical regions throughout the world. They show how rising and revisionist powers are putting pressure on our frontier allies--countries like Poland, Israel, and Taiwan--to gauge our leaders' commitment to upholding the U.S.-led global order. To cope with these dangerous dynamics, nervous U.S. allies are diversifying their national-security "menu cards" by beefing up their militaries or even aligning with their aggressors. Grygiel and Mitchell reveal how numerous would-be great powers use an arsenal of asymmetric techniques to probe and sift American strength across several regions simultaneously, and how rivals and allies alike are learning from America's management of increasingly interlinked global crises to hone effective strategies of their own. The Unquiet Frontier demonstrates why the United States must strengthen the international order that has provided greater benefits to the world than any in history.