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Americans live in a perpetual state of war. Although Congress has not formally declared war since 1942, U.S. presidents have repeatedly deployed troops overseas based on United Nations resolutions, congressional authorizations, or their own executive power—as President Donald Trump has in Iran.

But the power that presidents possess in matters of national security is not the product of an inevitable expansion of the executive authority. Congress builds the imperial presidency one step at a time. At key moments in U.S. history and in response to international threats, Congress has made deliberate decisions that have resulted in presidents being allowed to deploy troops and assert executive power over civil liberties.

Americans live in a perpetual state of war. Although Congress has not formally declared war since 1942, U.S. presidents have repeatedly deployed troops overseas based on United Nations resolutions, congressional authorizations, or their own executive power—as President Donald Trump has in Iran.

But the power that presidents possess in matters of national security is not the product of an inevitable expansion of the executive authority. Congress builds the imperial presidency one step at a time. At key moments in U.S. history and in response to international threats, Congress has made deliberate decisions that have resulted in presidents being allowed to deploy troops and assert executive power over civil liberties.

One of the most consequential moments came in the aftermath of the horrific attacks of 9/11. Just days after Americans suffered the deadliest-ever attack on U.S. soil, Congress passed the Authorization for Use of Military Force (AUMF) with bipartisan support. The measure placed the nation on a permanent wartime footing, greatly expanding the range of actions a president could take without seeking additional congressional approval. Although some skeptics succeeded in narrowing the scope of the authority granted by the legislation, it remained so broad that its effects continued to shape national security policy today.

Understanding the decisions Congress made in the weeks following 9/11 is as important as commemorating the tragedy itself. Those decisions shaped how the nation would respond to terrorist threats, the constitutional safeguards Congress was willing to sacrifice in the name of security, and the damaging consequences for the balance of power that resulted from elected officials’ efforts to protect the country.

The 9/11 attacks devasted the nation. The........

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