Until 11 September 2001, the Twin Towers in New York were a symbol of international trade and human potential. After the attack, the gaping hole in Manhattan became a symbol of terrorism. Two passenger planes, with an interval of a quarter of an hour, crashed into the towers just after the start of a working day. Less than one hour after the impact, the South Tower collapsed, followed by the North Tower. Although many people managed to leave the buildings in time, almost three thousand were killed. In addition, a hijacked plane struck the Pentagon and a fourth plane crashed in Pennsylvania. In the latter case, the passengers overpowered the terrorists, thus preventing the plane from reaching its target. The attacks were carried out by nineteen members of the Islamic terrorist network Al Qaeda, all from the Middle East.

The events that took place on 9/11 caused an enormous shock worldwide and had huge consequences. The United States regarded the attacks as a declaration of war and in turn declared war on terrorism. In search for Al Qaeda and its leader Osama Bin Laden, they invaded Afghanistan. In addition, the war against terror was used as an argument for attacking Iraq. Along with other Western governments, the Dutch cabinet of Balkenende also supported this attack, despite internal criticism. In Afghanistan, the Netherlands took part in a reconstruction mission, an effort that was controversial right from the start and which acquired a more military character than intended, due to the stubborn resistance of the Taliban.

Muslim extremism also crops up in the Netherlands, as demonstrated by the murder of filmmaker Theo van Gogh in 2004 and threats to politicians like Ayaan Hirsi Ali and Geert Wilders. The number of extremists – in the Netherlands mainly a phenomenon among young people – still appears to be small for the time being. The threat of terror became even more tangible with the bloody attacks in Madrid on 11 March 2004. Partly as a result of this, but also because of integration problems in practice, heated public debates arose about the danger to the Netherlands of fundamentalist Islam. However, there is at the same time a constant stream of initiatives at all levels of society (in neighbourhoods, at schools, in social organizations, in cultural-scientific circles) to stimulate the harmonious coexistence of different population groups.