The prophet Muhammad himself ordained decapitation as a routine procedure, Professor Timothy Furnish makes clear in this essay on Beheading in the Name of Islam (from the Middle East Quarterly, Spring, 2005):

The essay rewards reading in full. Here are some extracts:

Sura (chapter) 47 [of the Qur’an] contains the ayah (verse): “When you encounter the unbelievers on the battlefield, strike off their heads until you have crushed them completely; then bind the prisoners tightly.” …

Beheading has particular prominence in Saudi Arabia. In 2003 alone, the Kingdom of Saudi Arabia beheaded more than fifty people. This number included both Muslim and non-Muslim workers. Over the past two decades, the Saudis have decapitated at least 1,100 for alleged crimes ranging from drug running to witchcraft and apostasy. The Saudi government not only uses beheadings to punish criminals but also to terrorize potential opponents….  While outsiders may consider the Saudi practice barbaric, most Saudi executions are swift, completed in one sword blow. Zarqawi [the al-Qaeda leader killed by coaliti0n forces in Iraq in June 2006] and his followers have chosen a slow, torturous sawing method to terrorize the Western audience. …

The  beheading of the Americans, Daniel Pearl, Nicholas Berg, Jack Hensley, and Eugene Armstrong, were filmed and the videos distributed for world-wide television news.

Islam is the only major world religion today that is cited by both state and non-state actors to legitimize beheadings. And two major aspects of decapitation in an Islamic context should be noted: first, the practice has both Qur’anic and historical sanction. It is not the product of a fabricated tradition. Second, in contradiction to the assertions of apologists, both Muslim and non-Muslim, these beheadings are not simply a brutal method of drawing attention to the Islamist political agenda and weakening opponents’ will to fight. Zarqawi and other Islamists who practice decapitation believe that God has ordained them to obliterate their enemies in this manner. Islam is, for this determined minority of Muslims, anything but a “religion of peace.” It is, rather, a religion of the sword with the blade forever at the throat of the unbeliever.

The only part of this that we challenge is his phrase “for this determined minority of Muslims”. If Islam is a religion of war and violent conversion by order of their holy book, the Qur’an, then it is so for all Muslims, or else the alleged majority is Muslim in name only and not belief.