People do things when they're angry and scared that they live to regret. This is an everyday truth, but when the deeds are done by governments the consequences can be calamitous.
There's a small field of psychology, known as terror management theory, that studies this kind of behaviour. Its practitioners test people's responses when they feel their lives are under threat, such as after a terrorist attack. What they find is that at such times people cling ever more closely to their own worldviews (cultural, religious, nationalistic) and to those who share them, and turn ever more hostile towards members of out-groups who hold opposing views. Or as one of the leaders of this field puts it, we become intent on showing that "our god is better than your god and we will kick your arse to prove it".
This effect has been demonstrated hundreds of times in experimental settings, and in real-world scenarios too. In the days after the 9/11 attacks, which triggered an existential crisis among many Americans, patriotic fervour and church attendance in the US increased significantly, as did intolerance for dissenting opinions and calls for retaliation. George W. Bush, who reflected the national mood in his speeches and foreign policy plans, was suddenly the most popular president on record. Weeks later, the vast majority of Americans approved of his decision to invade Iraq.
In hindsight, with Islamic State now profiting from the disastrous aftermath of that campaign, many in America (and, it seems, almost everyone in Europe) think he and Tony Blair took the wrong course. Right now in Europe the mood of fear, anger and vulnerability is akin to that in the US after 9/11, which is to say that it is very easily manipulated. We are much more inclined at such times, when we are keenly aware of our mortality, to support symbolic aggressive action, because it draws us closer to our in-group and makes us feel more secure. But this sense of security is a psychological illusion, because in the long run such decisions more often than not come back to haunt us.
Before he sent British troops to Iraq in 2003, Blair compared international feet-dragging on the issue with the failure to act against Hitler after the 1939 Nazi occupation of Czechoslovakia. David Cameron has used comparable language to garner support for air strikes against Islamic State. This is a psychological upper-cut, designed to sidestep rational consideration of the risks of such action. Leaders down the ages have used similar tactics to drum up support for war by taking advantage of people's existential fears, a political no-brainer. It is one of the oldest tricks in the books, and we shouldn't allow it to catch us out.