They have struck before on a larger scale - with regularity - as documented e.g. by Clube and Napier. Much of their research focused on the long-past break-up of a very large comet and the periodic intersection of Earth's orbit with its remains - which has led to cometary showers, with their impact on societies in more ways than one, also leading among other things to religious developments - ideas of gods and their actions and judgments.
Historically, peoples have looked to their leaders to protect them from catastrophe - and when their leaders fail to do so, i.e. something happens that they simply cannot control, such as a rain of fireballs and meteorites exploding in the atmosphere, then a people will blame its leaders and get rid of them - often violently. This seems to be a basic feature of human psychology, one repeatedly seen in action throughout history.
Knowing this, the leaders have the need to reassure their people that they have things under control - historically, there have e.g. been systems of ritual and sacrifice. Nowadays, reassurances come in a different form: That the sky is watched, that major events only happen "once in a lifetime" (or, earlier, that such things simply couldn't happen - which was long the consensus), coupled with simplistic ideas of weapons and other solutions to take out the threat - solutions that will never be adequate if/when the time comes for real. People are only too happy to play along with such reassurances, to develop them and then to take them and run with them, since the alternative is not too pleasant - recognizing that there is no way to avert such disasters when they arrive.
A very recent book by a historian, "Comets and the Horns of Moses", discusses this whole subject, and much more connected to it. It goes into the history of cometary interaction with our planet - which has long seemed to follow cycles - and both how it has affected life on Earth and how humanity has responded to it - the social, cultural, and political dynamics involved, both in-between and during times of cometary disasters. Looking at the history and the present, it further goes into what seems likely to be coming up. I'd recommend it for the interested.
In the present time, one of several clues is the reported sightings collected by the American Meteor Society, which have increased roughly exponentially since 2005 - with 463 events on record for 2005, the increase accelerating year by year with 1628 for 2011 and then 2219 for 2012. Thus far this year - i.e. in one and a half month - there's been 322.