It depends what you mean by "threat".
I really struggle to take these "terrorist groups" very seriously, if I'm honest. We hear of their failed attempts to blow up aircraft, where one guy failed to set fire to his shoes before another passenger thumped him, then another guy failed to set fire to his underwear before other passengers beat ten bells out of him and sat on him. In the UK there were always periodic news reports of the police raiding a suspected terror cell (which inevitably meant a few young men with brown skin and beards being dragged away in the small hours of the morning) and nothing was ever heard of them again. If they were found guilty of something we should have known, and if they were released we should also have known.
... and all the while the rights of the people are eroded as the government is desperate to Be Seen To Be Doing Something, and so many of the people gladly give up their hard-won freedoms in a plea to Nanny State to Please Do Something and Protect Us From The Bad People, never really realising that there is no way Nanny State can possibly do such a thing.
I say the biggest threat to a civilised nation is the people of that nation who grow too comfortable, too complacent, and too distracted by the latest episode of Strictly Come Celebrity Cow Tipping On Ice With The Stars to stop and think about what is being done in their name. When people cast votes for high office based on someone's hair, or the color of their skin (during the 2008 election I was surprised at the number of times I heard variations of "it's about time we had a black President" and "I don't want no *racial slur* in the White House"), it's hardly surprising they don't get the results they wanted. When people turn politics into soundbites, where the Democrats all but eat babies for breakfast and the Republicans would sell their granny for dog food if it meant paying a few bucks less tax, the actual significance of the issues is lost.
We can blame those in office all we want but at the end of the day we get the government we vote for.