In US propaganda, and in most of the media that blindly adopted the jargon of the former, we first had the "Baathist diehards/dead-enders", then the "Saddam loyalists and foreign insurgents", now we have "a coalition of ex-Baathists, Sunni and Shi'a Islamic fundamentalists". (Well, we have the same in the Transitional [Puppet] Government.) The picture was always much more diverse, so it is useful to recap before introducing the latest development referred to in the title. (Note: I decided not to Google for the appropiate links, too many would be needed, and Dear Reader can Google too :-))
Militant resistance came first really from ex-Baathists, who are centered on Tikrit (regarding the recent false story of Izzat Ibrahim's capture, his influence beyond Tikrit was most probaly always bloated up). But very soon the casualties from reckless US methods led many Sunni Arabs to take up arms to carry out a vendetta for a relative (son, father, daughter, mother, wife, grandmother, cousin, niece second grade - not just what are immediate family members in the much more atomic American society). By May 2003 already, in places like Fallujah, this was further escalated by large-scale massacres (shooting at unarmed protesters) and attacks on tribal sheiks, by triggering tribal militias to take up arms.
In the meantime, two allied Sunni Islamic fundamentalist terror groups also turned active: Ansar-e-Islam, a Kurdish group (beyond the two mayor, US-allied groups with militias, there were several smaller ones, Ansar just one of them, but perhaps the nastiest); and al-Tawhid, led by the now (in)famous Zarqawi. Tough Zarqawi was born in Jordania and was a warlord in Afghanistan, who was allied to Bin Laden (portraying him as a subordinate member of al-Qaida was always a convenient but dangerous misconception), much or most of his murderous group is said to consist of local Iraqi fundamentalists. Further foreigners, mainly from Saudi Arabia, Jordania and Syria, are said to having had joined resistance groups in Fallujah by more than one non-US journalist's reports from there (I recall in particular the story of a Saudi sharp-shooter taking out a US sniper). But Fallujah, erroneously portrayed in most news reports (thanks to US propaganda) as a centre of well-off Saddam loyalists, has its own Islamic fundamentalist tradition, as a centre of Salafism (Fallujah is also called "city of hundred minarets"; and speaking of being well-off, it was also famous/taunted for its stench: lack of water, no canalisation...), which was persecuted under Saddam.
It should be noted that Fallujah isn't the only local centre of Sunni resistance, it was only the first and fiercest: those in Ramadi, Samarra, Baquba, and probably the Arab part of Mosul are also local in nature, and the first three forced the US military out to the same extent Fallujah already did before the Marines arrived (but not yet as fully as when the Marines were forced out).
The developing Shi'a Arab resistance didn't start with the Mahdi Army (see next paragraph), but became big with it. The Mahdi Army is most often mentioned in connection with Najaf (which is hostile territory to Sadrism but coveted prize as religious center) and the Baghdad slum of Sadr City, but more centres are in Kufa and lots of cities to the South of Najaf (in particular Amarah). The Mahdi Army is Sadrist in origin, but it is not a disciplined, strictly hierarchic army, more a bunch of armed gangs broadly reverent of Muqtada Sadr, its core already having been reverent to his father. (Some journalists reported recently from Sadr City that some commanders don't want to stop fighting even if Sadr tells them to, for example.) Also, probably the majority of its members are not fundamentalists, but people more attracted to the nationalist and socialist element in the Sadrist credo, as a number of journalists' reports about Mahdi Army fighters imply - like, some of them drinking beer. However, the Mahdi Army isn't the only big militia fighting the occupation: elements of the Marsh Arab Hezbollah - the militia that earlier waged a guerilla war against Saddam, but whose leader resigned from the IGC during the April 2004 US assault on Fallujah - also joined in. So did smaller groups to the North, in some Shi'a enclaves in Sunni-majority cities North of Baghdad (I got this from Juan Cole).
Further mention should be of 'common criminals'. I put it between marks because in some cases kidnapping is clan or tribal 'tradition'. Criminals come into the picture from three directions. One, early US propaganda often mixed up waylaying by common criminals with acts of the resistance that targeted the occupation army's supply lines. Two, Scott Ritter and at least one more report says that the Baathist fraction within the resistance used to pay criminals for risky attacks, just to learn for their own later attacks what tactics US units use in response. Three, kidnappers for ransom (who were very much active from last summer on, especially in the Shi'a South, but most of the Western press ignored them because the victims were Iraqis) piggybacked on the surge of kidnappings for political blackmail.
Now, finally, the latest community members of which took up arms against the ocupation seems to be Shi'a Turkomen - Juan Cole reports
Heavy fighting raged in the northern city of Tal Afar, which lies west of Mosul, again on Sunday, according to ash-Sharq al-Awsat. The population of Tal Afar is largely Turkmen Shiites, and it is unclear why the US military is fighting there. A US spokesman claimed that the city is a transit point for foreign terrorists slipping in from Syria. This could well be the case. But would Turkmen Shiites really give over their town to the control of Arab Sunni fundamentalists? Many Turkmen Shiites have given their allegiance to Muqtada al-Sadr, but no source mentions Mahdi Army as an issue there. It appears that the city, not just a handful of fighters, is resisting the US troops, though the Wall Street Journal spoke of the gunmen as a "cell". At least in the Sunni Arab cities, such as Fallujah, Ramadi and Samarra, it makes sense for Sunni nationalists and fundamentalists to ally with a handful of outside Sunnis who have come as volunteers. In Tal Afar, that scenario makes no sense. Local Tal Afar medical authorities said that two Iraqis were killed and 9 wounded in battles.
Whether we'll see larger and less extreme Kurdish groups joining, and whether all or most of these groups will be able to form a stable coalition, or not and hence will instead be a basis for the inevitable breakup of Iraq into what can be best described as city-states, we'll see.