Here is the big picture as I see it. Sociologically, al Qaeda, Tawhid, and Ansar al-Islam are all part of what Gilles Kepel calls “salafi-jihadism” and Marc Sageman calls the “global Salafi jihad”. Al Qaeda is bin Laden’s organization. Tawhid is Zarqawi’s organization and exists because Bin Laden didn’t want Jordanians in al Qaeda, since they might be working for pro-Western Jordanian intelligence. (I picked up this idea from an article at SAAG.org or Jamestown.org, not sure which.) Ansar al-Islam is the movement to bring about a Sunni Islamist state specifically in Iraq. It is not restricted to Kurds, but it was first formed in Iraqi Kurdistan.

“To start with, Zarqawi is the leader of Al-Tawhid, not Ansar Al-Islam, as John states.”
This is a quibble. While our knowledge of these groups is conjectural to some extent, it appears that a number have linked together, beginning with the establishment of the Kirma camp and developing further. These groups are now generally referred to, collectively, by the name of Ansar-al Islam and Zarqawi is generally referred to as the leader. Google gives thousands of references to this effect.

From that point of view, I can well believe that Zarqawi’s camp in Iraqi Kurdistan was left alone because the objective in Iraq was always going to be regime change, and the existence of the Ansar camp facilitated the cover story. As I recall, Zarqawi and ricin were the centerpiece of Colin Powell’s argument to the Security Council that Iraq might engage in WMD terrorism. I think they were a sort of substitute for Bin Laden and anthrax, the Iraqi connection to which is the real “forbidden truth”.