President Donald Trump's administration over the weekend ordered a second military strike on the Syrian government without asking for permission from Congress.

Congress has the sole constitutional authority to declare war, but most US military action since 2001 has been covered by sweeping legislation designed to cover actions against terrorist organizations linked to the 9/11 attacks.

But legal experts say attacking the Syrian government over the weekend stretches the framework of the law.

Few congressional checks remain on the US president's ability to start wars, and a congressman told Business Insider the legislature was "derelict in its duty" for allowing this.

Over the weekend, President Donald Trump's administration for the second time ordered a military strike on the Syrian government without asking for permission from Congress, and it could indicate the legislature has lost its ability to stop the US president from going to war.

The US Constitution, in Article I, Section 8, clearly states that the power to declare war lies with Congress, but since 2001 successive US presidents have used military force in conflicts around the world with increasingly tenuous legality.

Today, the US backs up most of its military activity using broad congressional legislation known as the Authorization for Use of Military Force. The joint resolution, which Congress passed in 2001 after the 9/11 terrorist attacks, allows the president to "use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on Sept. 11, 2001, or harbored such organizations or persons."

This has essentially become a carte blanche for the US president to fight terrorism wherever it rears its head.

But on Friday night, early Saturday in Syria, the Trump administration attacked Syrian targets in retaliation for an attack on a Damascus suburb the US says involved chemical weapons. Trump ordered a similar punitive strike a year ago, in April 2017.

At Harvard's Lawfare blog, the law professors Jack Goldsmith and Oona A. Hathaway summed up all of the Trump administration's possible arguments for the legality of the Syria strikes in an article titled "Bad Legal Arguments for the Syria Airstrikes."

The article concludes that the US's stated legal justification, that Article II of the Constitution allows the US to protect itself from attacks, falls short and that other legal arguments are a stretch at best.

Rep. John Garamendi, a California Democrat on the House Armed Services Committee who spoke with Secretary of Defense James Mattis hours before last weekend's strike, told Business Insider the strikes were probably illegal.

"The bottom line is I do not believe he has legal authority to conduct those strikes," Garamendi said.

Congress 'derelict in its duty' as Trump doesn't even try to get approval

The Laboon firing a Tomahawk land-attack missile. US Navy photo by Mass Communication Specialist 3rd Class Kallysta Castillo

Trump "could have and should have come to Congress and said these facilities and the use of poisonous gas is horrific, it is illegal based upon the international conventions, and I want to take military action," Garamendi said, adding that he thought "a limited authorization to do that would have passed Congress in one day" if it had been written in a concise, limited way.

But Trump did not ask for permission, and it shows the incredible power of today's US presidents to start wars.

"I think that Congress was derelict in its duty," Garamendi said. "Congress clearly has abdicated one of its most crucial functions, and that is the power to take the US into a war. The Constitution is absolutely clear, and it's for a very important reason."

Fred Hof, a former US ambassador to Syria who is now at the Atlantic Council, said that while there was some reason for Congress to allow the president leverage in where and when he strikes, the two branches of government still needed to coordinate.

"Most, maybe all, in Congress would concede there are circumstances in which the commander-in-chief must act quickly and unilaterally," Hof wrote to Business Insider. "But there are reasons why the Constitution enumerates the duties of the Congress in Article One, as opposed to subsequent Articles. I really do believe it's incumbent on the executive branch to consult fully with the Congress and take the initiative in getting on the same page with the people's representatives."

Lawrence Brennan, a former US Navy captain who is an expert on maritime law, told Business Insider "the last declaration of war was in the course of World War II," adding that Congress had "absolutely" given the president increased powers to wage war unilaterally.

Possibly illegal strikes create a 'window' for the US's adversaries

President Donald Trump with Russian President Vladimir Putin. Thomson Reuters

The US missile attack carried out over the weekend had questionable legality, but it wasn't even Trump's first time ordering strikes against Syria's government, as a salvo of 59 cruise missiles targeted a Syrian air base in April 2017.

Before that, the US attacked Libya's government forces in 2011. The US is also using the 2001 Authorization for Use of Military Force as justification for attacking Islamist militants in the Philippines, among other countries.

Garamendi said that by neglecting to request congressional approval, Trump had "given Syria, Russia, and Iran an argument that never should have happened." He said by opening an internal US argument over whether the strike was legal, Trump had committed a "very serious error" and "opened a diplomatic attack that could easily have been avoided."

Trump certainly did not start the trend of presidents ordering military action without congressional approval, and he has enjoyed wide support for his actions against chemical weapons use, but the move indicates a jarring reality — that the US president can go to war with thin legal justification and without even bothering to ask Congress.