President Trump celebrated the recapture of the Islamic State's last major stronghold Saturday as a "critical breakthrough" in the campaign against the terrorist group.

"With the liberation of ISIS's capital and the vast majority of its territory, the end of the ISIS caliphate is in sight," the president said in a White House statement.

The Syrian city of Raqqa has been the chief, and ultimately final, major city that the terrorist group held since they made their rampage across Syria and northern Iraq. The U.S.-led coalition drove them out of most of Iraq through 2016, culminating in a nine-month battle for Mosul that ended in July. Raqqa's liberation followed months later, after a siege process that began in November.

"One of my core campaign promises to the American people was to defeat ISIS and to counter the spread of hateful ideology," Trump said. "That is why, in the first days of my Administration, I issued orders to give our commanders and troops on the ground the full authorities to achieve this mission. As a result, ISIS strongholds in Mosul and Raqqah have fallen. We have made, alongside our coalition partners, more progress against these evil terrorists in the past several months than in the past several years."

That statement marked Trump's most-emphatic contribution to a debate about how much credit his administration should receive for the victories. Proponents of his policies emphasized the importance of relaxed rules of engagement that Trump's team implemented after he took office.

"He directed a tactical shift from shoving ISIS out of safe locations in an attrition fight to surrounding the enemy in their strongholds so we can annihilate ISIS," Defense Secretary Jim Mattis said in May.

Former Defense Secretary Ash Carter, who led the Pentagon at the end of former President Barack Obama's presidency, argued that the tactical change didn't reverse the strategic course that had been set.

"The plan ... was laid out two years ago, and has been executed pretty much in the manner and the schedule that was foreseen then," Carter told CNN.

The fall of Raqqa marks a moment, long-foreseen in American foreign policy circles, when the threat of ISIS as a land-holding group diminishes and world powers focus on jockeying for long-term strategic advantage in Syria. "The day after Raqqa falls is going to be the moment that Iran moves to try to oust the United States from the region," Sen. Marco Rubio, R-Fla., told the Washington Examiner in February.

Trump acknowledged that the U.S. has more work to do in Syria. "We will soon transition into a new phase in which we will support local security forces, de-escalate violence across Syria, and advance the conditions for lasting peace, so that the terrorists cannot return to threaten our collective security again," he said. "Together, with our allies and partners, we will support diplomatic negotiations that end the violence, allow refugees to return safely home, and yield a political transition that honors the will of the Syrian people."