Some Democrat and Republican lawmakers are criticizing President Donald Trump for carrying out airstrikes in Syria without getting Congress’ approval first.

The United States launched nearly 60 cruise missiles at a Syrian airfield April 6, reacting to Syrian President Bashar al-Assad’s recent chemical attack that killed more than 80 civilians.

"The Constitution says war must be declared by Congress," said Sen. Tim Kaine, D-Va., in a statement April 7. "I voted for military action against Syria in 2013 when Donald Trump was advocating that America turn its back on Assad’s atrocities. Congress will work with the president, but his failure to seek congressional approval is unlawful."

Sen. Rand Paul, R-Ky., issued a similar statement that read, "While we all condemn the atrocities in Syria, the United States was not attacked. The president needs congressional authorization for military action as required by the Constitution, and I call on him to come to Congress for a proper debate."

Whether it was constitutional for Trump to carry out these airstrikes in Syria without congressional approval is not an easy question to answer — especially because the Trump administration has not yet made public its own legal justification. We asked the Trump administration, and we’ll update this post if we hear back.

The Constitution says Congress has the power to declare war, but throughout the past 70 years or so, presidents have adopted a flexible view of their own constitutional role as commander-in-chief in order to engage in military action without congressional approval.

What we found is that in some cases people saying Trump needed congressional approval have gone too far, as U.S. Rep. Mark Pocan did in claiming that there is "no legal basis" for Trump's action. (PolitiFact Wisconsin rated that claim False.)

Yet the courts have not settled some constitutional questions about what is war and who gets to decide when the United States attacks. This story explains the complicated issue.

War powers evolution

For most of American history, the consensus view among politicians and legal scholars was that the Founding Fathers meant for Congress to have the "sole authority" in deciding to initiate any hostilities or war with another country. Although this remains the most common view among academics, actual practice has diverged significantly, said Andrew Kent, a law professor at Fordham University and an expert in the Constitution and national security.

"Hence, there is a kind of customary law of war initiation in modern times that has supplanted a long-held view, and the view of the Founders, about what the Constitution means," Kent said.

The last time Congress actually declared war was World War II. In 1973, to retain control over their constitutional authority, Congress passed the War Powers Resolution. The resolution required that, in the absence of a war declaration, the president must report to Congress within 48 hours of introducing armed forces into hostilities and must remove forces within 60 days unless Congress permits otherwise.

As of this writing, Trump is still within the 48-hour window to notify Congress and seek their approval.

But some legal scholars think the War Powers Resolution unconstitutionally restricts the president’s authority, and it hasn’t always stopped presidents from taking military action without congressional approval. For example, President Bill Clinton sent thousands of U.S. troops into Kosovo in the 1990s to participate in a NATO peacekeeping mission. Clinton never received congressional approval, yet he did not remove the troops after the 60-day mark.

Controversially, President Barack Obama took a broad interpretation of Congress’ decade-old authorizations for 9/11-related military action in order to justify airstrikes and other measures against the Islamic State, including in Syria, without obtaining further congressional approval.

Trump can’t easily use that same justification because the United States attacked Assad’s regime, not Islamic State fighters, wrote Harvard Law School professor Jack Goldsmith in an April 7 Lawfare article. But he might be able to find precedent in the Obama administration’s legal justification for the 2011 U.S. intervention in Libya.

The White House Office of Legal Counsel determined then that it would be consistent with the Constitution and the War Powers Resolution for Obama to take military action in Libya because it served the "national interest." Also, the operation was intended to have a limited scope and short duration, as opposed to a full-on war.

Goldsmith noted, however, that it will be difficult for the Trump administration to come up with a sufficient reason why attacking Syria serves the national interest beyond maintaining regional stability and protecting international norms against chemical weapon use. Interestingly, Trump said the airstrikes were in the national interest when he announced them April 6 in Florida: "It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons."

So over the past few decades, presidents have routinely asserted their authority to take military action without prior congressional approval. Congress has largely acquiesced, and the Supreme Court hasn’t weighed in in any significant way.

So reasonable minds can disagree over whether this trend is constitutional.

Although Trump felt justified to take this action in Syria without congressional approval, he had a different take as a civilian about four years ago, when Obama was mulling taking military action in Syria after a chemical weapons attack. Trump said then that Obama shouldn’t attack Syria, but if he does he needs congressional approval first.

"The President must get Congressional approval before attacking Syria-big mistake if he does not!" Trump tweeted Aug. 30, 2013.