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Like U.S. presidents before him, Donald Trump has found that Congress does not always bend to his will. Unlike many predecessors, Trump has turned to an expansive definition of “emergency” to bypass the legislative branch when he wants something done. There are several U.S. laws that grant broader authority to the president in special circumstances, especially when the president says national security is threatened. The latest example is Trump’s threat to impose tariffs on Mexico in a bid to stop migration.

1. What exactly is a national emergency?

The National Emergencies Act of 1976 allows the president to make a declaration that gives him special, temporary power to deal with a crisis. In the past, most such invocations have been related to foreign policy, like prosecuting a war or responding to a global trade threat. International concerns explain most of the currently active national emergencies. On a few occasions, however, presidents have used emergency declarations to further their domestic policy goals.

2. Who decides what qualifies as an emergency?

There’s a case to be made that that’s entirely up to the president. Emergencies have been declared during crises large (the Civil War, the Great Depression, the aftermath of the Sept. 11 terror attacks) and small (remember the 1970 postal strike?). The 1976 law that codifies the president’s broad authority to declare a national emergency also requires statutory authority for emergency actions. That law was passed as part of a sweeping set of legislation designed to restrain presidential powers after the Watergate scandal.

3. How is Trump using emergencies?

Frustrated that Congress wouldn’t fully fund his promised border wall along the U.S.-Mexico border, Trump in February declared a national emergency to reallocate about $7 billion for construction. (That use of presidential authority is being challenged in court.) Also, in his ongoing use of tariffs against China and U.S. allies, Trump has utilized a presidential power under the Trade Expansion Act of 1962 that lets him apply tariffs when he sees U.S. national security at stake. And citing a rarely used provision of the Arms Export Control Act, the Trump administration recently moved to bypass Congress and approve the sale of more than $8 billion in weapons to Saudi Arabia and other countries in the Middle East. That law permits immediate sales when they are “in the national security interests of the United States.”

4. How would tariffs on Mexico be justified as an emergency?

Stretching this emergency power even further, Trump threatens to impose tariffs on all goods from Mexico to force its government to change immigration policy. The president does have the authority to control economic transactions in a national emergency under the International Emergency Economic Powers Act of 1977. During the Iran hostage crisis, for example, President Jimmy Carter used the law to freeze assets of the Iranian government. But the law has never been deployed to directly hit imports with tariffs, according to the nonpartisan Congressional Research Service. If Trump carries through on his threat, he could face lawsuits from trade associations and manufacturers, such as the U.S. Chamber of Commerce.

5. Could Congress do anything to stop this?

Congress in March voted to terminate Trump’s border wall-related emergency declaration, but Trump vetoed the measure, and the House couldn’t muster the two-third supermajority needed to override him. It would likewise be difficult to reach veto-proof support for legislation to keep Trump’s Mexico tariffs from taking effect. Another option: Congress could act on legislation that would restrict some of the president’s authority when it comes to imposing tariffs or other economically punitive measures going forward.

6. What’s happened in previous fights?

Courts have limited the president’s emergency powers before, including in a 1952 Supreme Court decision in which the high court said President Harry Truman could not use an emergency declaration to seize steel factories to ensure production during the Korean War. But since laws governing emergency authority have changed, some experts say it would be difficult to predict how the case would be viewed by the current Supreme Court, in which Republican-appointed justices hold the majority.