A series of deadly explosions rocked Brussels Tuesday, killing at least 31 people and injured 200 at Belgian capital's Zaventem airport and the Maelbeek metro rail system.
Officials raised the city's terror threat level and shut down public transit after the blasts.
The Islamic State claimed responsibility for Tuesday's attack, according to a post by affiliated Amaq Agency.
U.S. government agencies said the terrorist group's claim of responsibility appears authentic,
"It was really only a matter of when, not if, the next attack would strike Europe," said Clarke. "The city has become a petri dish for criminality, weapons trafficking and Islamic militancy."
Clarke said the threat to Europe is much higher than in the United States, and today's attack signals a "new normal" for the European Union.
"This is a very skilled enemy," said Clarke. "The attacks show a very high level of sophistication and they are actively recruiting.in France, the UK, Belgium and other Western nations like Finland, Denmark and Sweden. So the problem is very widespread and pervasive."
"More attacks this year are highly likely.Security services are overwhelmed across Europe and in the United Kingdom and they will simply be unable to keep up with the demands on their resources," said Nawaz.
"This will cause political pressure and a populist nationalist backlash as politicians take tough measures just to show they're doing something," he said.
Nawaz also said the terror cells seem to have the upper hand in Europe right now and the dynamic does not seem to be shifting that quickly.
"Jihadists want civil war in Europe between Muslims and non-Muslims," said Nawaz. "But we must not give into fear, otherwise jihadists would see such a result as a victory as they did in the Madrid bombings."