President Trump justified the launch of 59 Tomahawk cruise missiles on a Syrian air base Thursday night as being in the “vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.” He did not ask for Congress’ authorization to carry out the strikes.

Ordered in retaliation for a horrific Syrian sarin gas attack on civilians Tuesday, the strikes came on the 100th anniversary of the day the U.S. declared war on Germany and entered World War I. The U.S. has formally declared war 11 times in its history, but the last time was during World War II.

Trump ordered the Syria strike under the War Powers Resolution, which says a president has to report to Congress within 48 hours if the U.S. armed forces are introduced into a conflict. It’s a law that was enacted in 1973 to restore Congress’ role in authorizing force in response to the lack of a formal war declaration in the Korean and Vietnam wars. Under the law, troops cannot stay for more than 90 days unless Congress approves.

Today, American forces are still operating under the authorization for the use of military force that President George W. Bush requested after the September 11 attacks in order to fight countries or groups connected to the attacks.

Regarding the Syria strikes, the White House said that about two dozen members of Congress were notified and briefed while the strikes were underway, but some want Trump to seek congressional approval. “Assad is a brutal dictator who must be held accountable for his actions,” said Sen. Tim Kaine, Democrat from Virginia. “But President Trump has launched a military strike against Syria without a vote of Congress. The Constitution says war must be declared by Congress.”

“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,” said Sen. Rand Paul of Kentucky.

A true declaration of war would give the president broad legal authority, such as the ability to stop exports of agricultural products, control transportation systems, and order manufacturing plants to produce weapons — and even seize the plants if they refuse. President Truman skirted Congress when he sent troops to Korea in 1950 without seeking a declaration of war, eventually numbering 1.8 million U.S. service members. In the early days, he referred to the troop introduction as a “police action.” This set a precedent for future conflicts.

But since 9/11, the definition of “war” has become more vague and lacks the geographical restrictions it used to. Before a joint session of Congress on September 20, 2001, President Bush said, “Our war on terror begins with al-Qaida, but it does not end there. It will not end until every terrorist group of global reach has been found, stopped and defeated.”

In August 2013, President Obama drafted legislation for Congress to grant authorization of military force in Syria in response to a chemical weapons attack. It was not approved. Again, in February 2015, five months after the U.S. began launching airstrikes in Syria, Obama asked Congress to authorize force against the Islamic State group. It didn’t approve the authorization. In 2014, Rand Paul introduced a formal declaration of war against ISIS. It was not passed. For the 2011 strikes in Libya, the Obama administration argued it didn’t need authorization because the air campaign was part of an international coalition.

Rep. Barbara Lee, the only member of Congress who voted against the 2001 Authorization for Use of Military Force, tweeted in response to the strikes in Syria: “This is an act of war. Congress needs to come back into session & hold a debate. Anything less is an abdication of our responsibility.” She also said that the strikes were beyond the scope of the 2001 authorization that Congress granted Bush. Lee has previously introduced legislation to repeal the Bush-era authorization of force.