Can the President Attack Another Country Without Congressional Approval?

During a press conference on Tuesday, January 7th, President-elect Donald J. Trump refused to rule out the possibility of using military force to invade and control the Panama Canal and Greenland. While Trump has a history of saying things in an attempt to intimidate, provoke a reaction, or gain the upper hand in negotiations, his refusal to dismiss potential military action that many see no justification for raises questions.

Can the President of the United States (POTUS) start a war without Congressional approval? As Commander-in-Chief of the U.S. military, how much control do they have? As you might expect, the answers aren't simple.

No More Kings

The Founding Fathers wanted to avoid giving one person the authority to plunge the new nation into a war. While the office of the president was crafted with George Washington in mind, the creators of the Constitution realized that those who followed might lack his virtues and humility. This was considered well-established constitutional law until around 1950. However, the creation of the United Nations and changes in international law have made declarations of war somewhat obsolete. The U.N Charter prohibits declaring war for territorial gain, which is why most countries try to avoid appearing as the aggressor (the war in Ukraine notwithstanding).

It is still unequivocally true that only Congress has the power to declare war. They've done so 11 times, with the last instances occurring during World War II. It's understandable if you're scratching your head remembering Korea, Vietnam, Afghanistan, and other conflicts that sure seemed like wars. While Congress has only declared war 11 times, they've also approved 108 "military operations" through resolutions. This is where it gets complicated.

In 1950, President Harry Truman side-stepped Congress by going directly to the United Nations — then five years old — to authorize military action in North Korea. The U.N. did authorize military action, and Truman used U.S. troops to carry out the "police action."

However, perhaps the closest modern-day analogy to any potential military deployment in Greenland or Panama would be Operation Desert Storm. In 1990, President George H.W. Bush believed a military response was required after Iraq invaded Kuwait. While ultimately Congress declared a joint resolution authorizing the deployment of troops, Bush maintained that he had the right to deploy troops regardless and would have done so even if it led to his impeachment. Bush felt it was vital to U.S. economic and security interests to protect Kuwait. He ascribed to the view that Article II gives the president broad powers in deciding whether to commit U.S. forces overseas.

Ultimately, a Congressional resolution rendered the issue moot. But it's left a gray area regarding when a president can oversee the deployment of troops even without a declaration of war or Congressional resolution.

Congressional Action and Wartime Powers

As Commander-in-Chief of the U.S. military, the president makes all final military decisions during wartime. This includes the following:

• Deploying troops
• Invading hostile countries
• Sending spies into enemy territory
• Using nuclear weapons

As of 2025, 31 U.S. presidents had served in the military before taking office. Regardless of their experience, none have ever made a formal declaration of war on their own.

Congress has also expanded and retracted the wartime powers of the president. In 1941, the First War Powers Act gave President Franklin D. Roosevelt temporary control of many Congressional powers to enhance the efficiency of the war effort. These were repealed after the end of World War II.

The War Powers Resolution of 1973 (referred to as the War Powers Act) went in the other direction. It limits what the president can do with military forces if lacking Congressional approval. U.S. forces sent into combat are limited to 90 days without a Congressional resolution, 30 of which must be spent on their withdrawal.

While the War Powers Act limits prolonged engagements, it also grants the president the authority to "respond to attacks and other emergencies." This was the right Trump claimed in 2017 after ordering missile strikes against a military base in Syria without explicit Congressional approval.

More leeway was given to the Executive Branch through 2001's Authorization for the Use of Military Force. This joint resolution by Congress allowed President George W. Bush to attack any nation or group he considered responsible for the terrorist attacks on 9/11. This was also used by President Barack Obama in 2014 against the Islamic State.

The Potential Targets

There's no evidence that the U.S. has suffered any recent attacks from either Greenland or the Panama Canal. This hasn't stopped Trump from listing them as potential conquests. There are reasons the rest of the world might have issues with this.

Greenland

Greenland is an autonomous territory of the Kingdom of Denmark. It became a Danish state in 1814 and was fully integrated as an amt (county) in 1953. In 1979 Greenland was granted home rule.

The U.S. has long had a friendly relationship with Greenland. It established military bases in Greenland during World War II and still maintains the rent-free Pituffik Space Base, formerly known as Thule Air Base. The U.S. made an offer to buy Greenland from Denmark in 1946 for $100 million. The offer was not accepted.

Denmark is a fellow member of NATO. It's difficult, if not impossible, to find legitimacy for any claims the U.S. makes for attacking Greenland. While Congress could declare war at any time for any reason, doing so against long-time allies for purely economic reasons would be unpopular, to say the least.

The Panama Canal

Originally started by French interests, the Panama Canal connects the Pacific Ocean and the Caribbean Sea. The ingenious 51-mile-long system of locks has been in service since 1914. This includes expansions made to allow larger ships to pass through as well.

The U.S. took over construction in 1904 and was in control of the Canal until 1979, when it entered into a partnership with Panama. Full control of the canal was passed to Panama in 1999.

Is War on the Horizon?

Despite Trump's adamant assertions that the U.S. would be in the right to attack both targets, it's difficult to find justification in either instance. Claims that China is taking over the Panama Canal, thus making it a matter of national security, are unsubstantiated at this point. While Chinese companies have been buying infrastructure along the canal, they have no control over setting prices for U.S. ships, for example.

It's possible that Trump could strike first and ask permission later. Given that Republicans control both the House and the Senate may embolden him, but it's unlikely the rest of the world would sit by idly to what would appear to be unprovoked attacks. Whether or not Congress would support the actions and prolong how long military forces could be used with resolutions is unknown.

Related Resources

• Implied Executive Powers (FindLaw and the U.S. Constitution)
• What Is the War Powers Act? (Findlaw's Law and Daily Life)
• The Panama Canal and the Torrijos-Carter Treaty (U.S. State Department)