The Constitution gives Congress alone the power to declare war, but modern presidents have asserted broad authority to use military force.
Congress has done little to push back, including last week, when lawmakers voted down a resolution to halt President Trump's military action against Iran.
Democrats have sparred with Republicans and the administration over the legality of the strikes.
"We shouldn't be at war without a debate or vote," said Sen. Tim Kaine, D-Va. "That is what the framers intended."
"We've overcomplied with the law and what it requires," Secretary of State Marco Rubio told reporters after briefing lawmakers last week. "This is an action by the president to address a real threat."
The Constitution says the president is commander in chief. It also says the power to declare war rests with Congress. But making that delineation in practice has proved complicated and contested.
To understand how that happened, it helps to trace the evolution of the debate over war powers, beginning in the early days of the republic.
How tension between Congress and the president evolved
Columbia University law professor Matthew Waxman, an expert on war powers who served in several positions in the administration of George W. Bush, says this separation of power between the executive and legislative branches in this space did not create much tension at first.
"There just really wasn't much of a standing force," Waxman says. "The president would have to go to Congress to fund military forces in order to wage military campaigns abroad."
And that's what presidents did, up through President Franklin D. Roosevelt in World War II, the last time a president asked Congress for a formal declaration of war.
The U.S. emerged from that war as a global superpower — a superpower with nuclear weapons.
"So these are fundamental changes in the nature of war, the role of the United States in the world and the relationship between Congress and the president," says Yale University law professor Oona Hathaway, who has advised the State Department on international law during several administrations.
This shifting relationship was tested in 1950 when President Harry Truman deployed troops without approval after communist North Korea invaded South Korea.
"That ended up being a three-year, grueling war," Waxman says. "It really stands out as a high-water mark of presidential unilateralism."
So in 1964, President Lyndon B. Johnson did ask Congress to authorize escalating involvement in Vietnam after U.S. ships were attacked in the Gulf of Tonkin.
But President Richard Nixon's secret bombing operation in Cambodia starting in 1969 sparked congressional hearings and the 1973 War Powers Resolution.
"What Congress was really trying to do there was put itself back in the game, to assert its constitutional role," Hathaway says.
What is the War Powers Resolution?
The War Powers Resolution requires that Congress be consulted in advance "in every possible instance." The law also requires the president to notify Congress within 48 hours of introducing U.S. forces into hostilities. It additionally gives Congress a tool to halt U.S. involvement by voting on a resolution to end military action, subject to presidential veto.
The same law requires the president to pull back within 60 days if there has been no congressional authorization.
What can make things murky, Hathaway says, is that Congress declining to order a withdrawal is not the same as authorizing the use of force.
"It's sometimes misunderstood as giving the president a blank check for 60 days," Hathaway says.
Presidents have largely complied with notifying lawmakers and have gone to Congress to authorize wars in Iraq and Afghanistan. And it has been understood that the Constitution empowers presidents to act quickly in an emergency.
But presidents have also pushed the boundaries of their power to take action in certain circumstances, interpreting words like "hostilities" and "imminent" threat to fit their purposes.
Without Congress, strikes were ordered by President Bill Clinton in Kosovo, President Barack Obama in Libya, President Donald Trump in Syria and President Joe Biden in Yemen.
Why presidents have expanded their power
But Waxman and Hathaway say Trump is now taking a greater constitutional leap.
"Starting war in the Middle East that's now involving more than a dozen countries is war in the constitutional sense," Hathaway says. "The nature, scope and duration of this conflict is extraordinary."
Hathaway says Trump could have asked Congress for an authorization of force in anticipation of possible intervention, without sacrificing the president's ability to move quickly when the time to act came. She also says the actions raise not just constitutional questions but also compliance with international law.
Hathaway and Waxman note that the courts have mostly avoided weighing in on debates over war powers, leaving Congress to guard its own authority.
"James Madison in the Federalist Papers describes checks and balances as ambition checking ambition," Waxman says. "We've seen presidents generally asserting prerogative and Congress being pretty passive and acquiescent. Oftentimes, it's preferable for members of Congress to sit back and then either support the president or criticize the president depending on how those operations go."
And for now at least, that is the position of most Republicans and a few Democrats in Congress, but that calculus could change if this conflict stretches on and the sacrifices it requires grow.
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