Siebelt, a Dutch investigative journalist, ends his book De 4th Wereld Oorlog with a question: "Who will be next?" The reference is to the murders of Pim Fortuyn and Theo van Gogh.

Volkert van de Graaf, a leftist animal rights activist, murdered prime ministerial candidate Pim Fortuyn while radical Islamist Mohammed Bouyeri killed filmmaker Theo Van Gogh. Though coming from different backgrounds, their actions point to the commingling of leftist and Islamist ideologies in Holland and how the two in their separate ways struck at the core of liberal Western beliefs. Although both young men were educated in Holland, a country renowned for openness and social tolerance, they both relied on murder to silence the opponents of their totalitarian beliefs.

Siebelt, the Dutch-born son of an American serviceman who refers to himself as a "war baby," is a specialist on the subject of left-wing and Islamist terrorism. His book documents how leftists have exploited civil rights and free speech legislation to undermine the legal system and intimidate security agencies, forging a path now being trod by radical Islamists who operate with near impunity. This "unholy alliance"—as David Horowitz calls it in the title of his book on this subject in the United States—directs its energies at undermining Dutch democracy. The breakdown of the rule of law under a barrage of legal challenges has enabled Islamist threats to society—such as Mohammed Bouyeri—to remain on the streets despite having been on the security services' radar for years. It was this judicially enforced Dutch tolerance that allowed the taps on Bouyeri's telephone to be removed just days before he butchered Theo Van Gogh.

De 4th Wereld Oorlog is of great value in tracing the radical Left's role as a facilitator of the Islamist movement, a topic with echoes far beyond the Netherlands. Many of the groups documented by Siebelt (such as George Soros's Open Society Institute, Amnesty International, and the Trans National Institute) are based both in Europe and the United States and are major players in the "fourth World War" effort to undermine the West.