(CNN) President Donald Trump and President Barack Obama have now both ordered airstrikes in Syria, but there are key ways that Thursday's strike was different from the previous military strikes there.

The airstrike Trump authorized Thursday hit a Syrian government airbase in response to the chemical weapons attack carried out earlier this week.

It's a step Obama was unwilling to take, at least without congressional approval, as Obama elected not to strike Syrian President Bashar al-Assad's regime in 2013 after a chemical attack crossed his "red line."

But Obama did launch airstrikes in Syria a year later , as the US began a military campaign against ISIS in Iraq and Syria. The nearly three-year war against ISIS has led to a steady stream of US bombings from manned aircraft, drones and missiles fired from warships.

The Trump administration continued Obama's bombing campaign against ISIS when Trump was inaugurated, as US-backed Syrian rebels prepare an offensive on Raqqa, the terror group's stronghold in Syria.

On Thursday, US warships launched 59 Tomahawk cruise missiles at a Syrian government airbase, the first time the US has directly attacked the Assad regime in the country's six-year civil war.

The situation on the ground in Syria is incredibly complex, between government forces, US-backed Syrian rebels, Kurdish fighters, ISIS militants and foreign fighters from Russia, Iran, Turkey and elsewhere.

While Russia is aligned with the US in the fight against ISIS -- and Trump has previously suggested the two countries should work together to defeat the militant group -- Russia has helped prop up the Assad regime militarily. There were Russians at the base the US struck, according to a US defense official.

One of the concerns about striking Syria from many US lawmakers is that it risks starting a proxy war -- or worse -- with Russia.

Another key difference between Trump's strikes against the Assad regime and the ISIS airstrikes is the legal authorization for carrying them out.

Both the Obama administration and the Trump administration have relied on the war authorization that Congress passed after the September, 11, 2001 attacks to fight al-Qaeda across the globe.

Both administrations have argued that ISIS is an offshoot of al-Qaeda, but the Assad regime is unconnected to that, and lawmakers were quickly calling for a new war authorization for the latest strikes late Thursday evening.

"Assad is a brutal dictator who must be held accountable for his actions," Virginia Democratic Sen. Tim Kaine said in a statement. "But President Trump has launched a military strike against Syria without a vote of Congress. The Constitution says war must be declared by Congress."

When Obama considered launching airstrikes in Syria against 2013, he decided to go to Congress before striking, and the resistance he faced there convinced the president not to hit Assad.

The Trump administration, however, said Thursday that the airstrikes were not a change in Syria policy.

"I would not in any way attempt to extrapolate that to a change in our policy or posture relative to our military activities in Syria today," Secretary of State Rex Tillerson said. "There has been no change in that status."