Chapter 3 examines how America’s turn toward continental and overseas expansion after the War of 1812 reshaped constitutional war powers through evolving practice. As the United States grew more secure from invasion yet more ambitious abroad, strategies of territorial acquisition and commercial protection strained inherited war-powers understandings developed for territorial defense. Even modest growth in peacetime forces, combined with executive control over diplomacy and command, broadened the President’s ability to create what Hamilton had called an “antecedent state of things” that narrowed Congress’s freedom of action. The chapter begins with the Florida campaign under the Monroe Administration, where cross-border operations, framed as self-defense, provoked constitutional disputes over presidential authority in undeclared conflict. It then turns to presidential threats of force, focusing on the Monroe Doctrine as a major assertion of executive power to define interests and signal commitments even when only Congress could authorize war. The Mexican–American War provides a central case study, highlighting constitutional questions about operational control, manpower, war aims, and governance of occupied territory, beyond the declaration itself. The chapter concludes with maritime operations and naval diplomacy, showing how distance and slow communications encouraged executive initiative while prompting episodic but consequential congressional oversight.