When our forefathers wrote the U.S. Constitution, they determined that Congress would have the job of declaring war, but that the president would have the power to take emergency action if the country was under attack.

The framers intended to provide the Commander in Chief a way to swiftly respond to security threats. But a new book by Chris Edelson, assistant professor at American University’s School of Public Affairs, shows through historic records that U.S. presidents have tested, pushed, and increasingly overstepped the limits of their emergency powers, especially in recent years.

Edelson explains four ways American presidents have abused their emergency power, particularly in the post-9/11 era:

The War On Terror
After 9/11, Americans became afraid and often looked to the president to defend the nation, said Edelson. Supporters of broad presidential power exploit this context in order to argue it is actually dangerous to set limits on the president’s power. Congress in the post-9/11 era isn’t pushing back on executive power as much as it should, says Edelson, and presidents aren’t necessarily seeking approval from Congress when they decide to act. The War on Terror further complicates this as it has no clear end and no clear target, making it harder to define the parameters of the president’s emergency powers.

Interrogation Tactics
Executive overreach occurred when it came to questioning terrorist suspects including through waterboarding and torture at Guantánamo Bay. Bush administration lawyers concluded in secret memos that anti-torture laws either didn’t apply or that the president could simply set them aside at his discretion, ordering whatever interrogation methods he believed were necessary, whether or not such methods were prohibited by criminal law.

Phone and Email Surveillance
President Bush first signed a secret order in October 2001 authorizing the National Security Agency to eavesdrop on U.S. citizens and foreign nationals in the United States, despite criminal law prohibiting such surveillance without a warrant. The Bush administration claimed the program was designed to monitor the phone calls and other communications of people in the United States believed to have contact with suspected associates of al Qaeda and other terrorist groups overseas, according to two former senior administration officials.

Military Strikes Against Libya and ISIS
More recently, Edelson says President Obama’s military strikes against Libya and ISIS exceed constitutional limits. As a candidate for president in 2007, Obama recognized that presidents could not order the use of force unless the U.S. was attacked or faced with an imminent attack. However, as president, Obama ordered military action outside of this emergency context.

Edelson says that presidents pushing the limits of executive power when it comes to national security is not entirely new. Abraham Lincoln, for example, suspended habeas corpus during the Civil War, though he ultimately secured congressional authorization to do so. President Harry S. Truman ordered the Secretary of Commerce to seize and operate the country’s steel mills to produce weapons during the Korean War— a move the Supreme Court declared unconstitutional. What’s different since 9/11 is that presidents have successfully exercised power unchecked by meaningful legal limits. Congressional acquiescence plays an important role in allowing this to happen.

Edelson’s research has been an important contribution to the national dialogue on presidential power. His book, published by the University of Wisconsin Press, is called Power Without Constraint: The Post 9/11 Presidency and National Security. It draws from excerpts of the Constitution, Supreme Court opinions, Department of Justice memos, and other primary documents.