Just back from a trip to the Middle East to assess the Islamist terrorist threat, U.S. Rep. Michael McCaul shared this frank observation with an audience at George Washington University:

“A worldwide terrorist exodus is underway, and we are woefully unprepared to deal with it.”

The Texas Republican, chair of the House Committee on Homeland Security, expressed grave concerns about the ability of the United States and its allies to prevent future attacks.

“Extremists are going forth to all corners of the globe to establish safe havens, to inspire followers and to advance a mortal threat against our people and our way of life,” Mr. McCaul said Thursday at a discussion on the radical terrorist threat to the West.

The event—timely as news reports emerged that morning of a downed Egyptian airliner—was held at the Jack Morton Auditorium and hosted by GW’s Center for Cyber and Homeland Security and the Program on Extremism. The center’s director, Frank Cilluffo, moderated.

Mr. McCaul said the state of Islamist terror has reached a new phase, one in which terrorists have gained unprecedented momentum.

“We are not winning this war,” Mr. McCaul said. “Violent extremists are not on the run, as the president says. They are on the march.”

Syria and Iraq are home to the largest convergence of Islamist terrorists in modern history, the congressman said. He added that more than 40,000 aspiring jihadists have entered the conflict zone, representing a larger fighting force than some countries have, including Denmark and Norway.

The result, according to Mr. McCaul, is that the Islamic State—the terror group known as ISIS—is more dangerous than al-Qaida when Osama bin Laden was at its helm. ISIS, he said, has been linked to nearly 90 plots to attack Western targets since 2011.

“This new generation of terrorists has franchised their violence by crowdsourcing attacks through social media and recruiting through tweets,” Mr. McCaul said. “These tactics represent a lethal evolution in our enemies’ ability to spread destruction.”

The depth of the threat was made clear, he said, during his trip this month to the Middle East, which included stops in Europe and northern Africa. Part of a bipartisan congressional delegation, Mr. McCaul met with leaders to examine the spread of Islamist militant groups.

“The upward trend of terror is tied to a proliferation of safe havens,” Mr. McCaul said. “Today there are more terror sanctuaries worldwide than we’ve ever seen,” with ISIS affiliates in nearly 20 countries from Algeria to the Philippines. Of growing concern is Libya, the second-largest ISIS stronghold, where Mr. McCaul said ISIS fighters are plotting Western attacks.

Meanwhile, the FBI is investigating 1,000 homegrown terror cases across all 50 states—80 percent of them ISIS related, he said.

Mr. McCaul was blunt in his assessment, stressing that the United States has not committed the resources needed to thwart the terrorists. The challenge is great, he acknowledged, as terrorists have become harder to track.

“Gone are the days of bin Laden, when terrorists plotting used caves and couriers,” he said. “A new generation of terrorists are using encryption, encrypted applications, to inspire others and to conspire with each other online and across borders, all while evading detection,” as with the recent attacks in Paris and Brussels.

Mr. McCaul offered his views on what it would take to defeat the terrorists: a military strategy to thwart terrorists on the ground, relying on local forces when possible; a political strategy to deny them the chance to reemerge; and a counter-narrative “to defeat them in the war of ideas” in part through counter-messaging campaigns.

During a question-and-answer session with the audience, Laura Khor, a visiting fellow with the American Leadership & Policy Foundation, asked about the challenges communities face in combating violent extremism and the ability of the government to help.

Mr. McCaul said military spending far surpasses resources put toward prevention tactics like community building. He added that through oversight and legislation Congress is working with the Department of Homeland Security to help communities spot the signs of radicalization and counter it.

With the world depending on American leadership, Mr. McCaul said, the United States must send a clear message to its enemies: “As you are plotting to bring terror to us, we are plotting to bring justice to you.”