|Soldiers from the 10th Bn prepare for exercises |
at their local headquarters in Mehriz, Yazd
The origin of the Imam Hussein battalions lie in the assessments conducted by now-Commander of the Islamic Revolutionary Guards Corps (IRGC) – General Mohammed Ali Jafari – during his time at the IRGC's Strategic Studies Center. By 2005 he concluded that the Basij were the 'backbone' of the IRGC, and recommended strengthening their combat-role by increasing coordination between them and the IRGC's Ground Forces (IRGCGF). 
Following his appointment to the position of IRGC commander in 2007, he initiated a series of reforms in pursuit of this objective. The result was the now-well known 'Mosaic Doctrine', which has been described elsewhere in great detail, but which – for our purposes – can be summed up as a province-centric organization that subordinates Basij combat operations to the IRGC when confronting 'hard threats' (i.e. interstate or conventional war), semi-hard threats (intrastate or unconventional war), and 'soft threats' (cultural, political and ideological contests).
This objective mandated new and reorganized Basij formations, which ultimately included Ashura, al-Zahra, Kosar, Beit al-Moqdas, and of course, Imam Hussein Battalions. The first IH Bns were created in 2008 and were operational by 2009.  
Within these new categories of Basij, Imam Hussein battalions are geared toward confronting the 'hard' end of the threat spectrum. They are light-infantry battalions assigned to the nearest IRGCGF division or brigade in their province. Conceptually, it is more accurate to think of them as reservists rather than a classical militia.
This helps explains why IRGCGF units generally have much smaller garrisons than their Artesh equivalents. Instead of a large standing-force, these units are comprised of an active-duty cadre that tends to the unit's peace-time duties and mans its high-density weapon systems (like tanks), but whose majority combat-strength comes from reservists mobilized during wartime.
In January 2010, the deputy commander of the IRGCGF Brig. Gen. Abdullah Iraqi claimed that IH Bns were the “most elite, and most trained” of all Basij units and contrasted their “military orientation” with the majority of Basij, who are geared toward confronting soft-threats.
In March 2012, the commander of one of the earliest IH Bns said that their primary mission is to “deal with hard threats and wars.”
Comments by Gen. Jafari in September 2012 further clarified the IH Bns' role relative to other Basij units. The former constitute front-line units that are functionally subsumed into their parent brigade or division during mobilization, while Ashura and al-Zahra battalions are true militia forces designed to support or free-up these first echelon forces by covering rear areas, or providing post-war stability.
Their potential employment in internal unrest is unclear. In the same January 2010 interview noted above, Brig. Gen Iraqi was asked about employing them against insurgents in Iran's south-east (Baluchistan) and north-west (Kurdistan). The IRGCGF deputy commander indicated that such a decision was the responsibility of the IRGC General Staff and thus beyond his purview, but conceded that should the IRGCGS made such a decision, there would be plenty of use for these battalions in combating insurgents.
Similarly, it is unknown whether any Imam Hussein battalions as such participated in suppressing the 'sedition '88' (aka, the 2009 election). Generally speaking, these battalions would not be the first to mobilize to counter such a threat, only being activated should the existing units (Imam Ali battalions) prove insufficient.
Outside of their primary role, these units have been mobilized to perform disaster relief operations, such as during the winter storms of 2013/2014 that paralyzed the country's north. They have also tasked with providing rudimentary military training/familiarization to Basij who themselves are not military-oriented. 
assigned to an IH Bn
Generally speaking, the Basij operate at four levels. The first is the province itself and is the fundamental operational unit. These are divided into Areas, which roughly correspond to the county-level, though some counties are divided into multiple Areas (ex: Ahvaz County, Khuzestan). Each Area is further divided into Zones (towns/villages), which in turn are composed of Bases (neighborhoods). Bases are administrative offices, not actual garrisons, and are often found in mosques or other neighborhood hubs and gathering places.
Imam Hussein battalions operate at the Base or Zone-level meaning that their offices are found in villages or neighborhoods within towns.  These offices are independent from non-combat Basij, and would not be expected to share facilities with them.
Although battalions are frequently identified as from this Zone, or that Area, it must be remembered that their commanders (including the commanders of these Zones and Areas themselves) are active-duty IRGC officers from the local brigade or division and do not constitute an independent chain-of-command. Bases, Zones and Areas should be thought of as administrative bodies for peace-time administration.
In September 2012, Gen. Jafari noted that more than 450 IH Bns had been created across the country. This corresponds to an average of 15 per province, and a nationwide strength of around 100,000 reservists (battalion strength described below).
However, since each province varies in size, population, and security requirements, these 450 are unlikely to be evenly distributed across the country. For instance, in Khuzestan (one of the more heavily garrisoned provinces) at least 25 are subordinate to the 7th Vali-Asr Division. In Yazd, during a gathering of 10,000 Basij from around the province, IH Bns constituted slightly less than one-third of the total number, which translates to around 12 known battalions subordinate to the 18th 'al-Ghadir' Infantry Brigade.
|2nd Imam Hussein Bn, subordinate to the IRGC's 18th Infanty Brigade, Yazd|
Imam Hussein battalions have both a numerical and nominal designation, though nomenclature for representing these designations varies widely. Sometimes the indicator-term 'operational' or 'Basij' is infixed somewhere in the name.This is made all the more confusing when other battalions are named 'Imam Hussein', but are not Imam Hussein-type battalions. Such are the challenges of drawing from a finite vocabulary of religious symbology.
According to comments made by the Deputy Commander of the Basij Ali Fazli, in November 2013, Imam Hussein battalions have a strength of 234 personnel. This is largely consistent with open-source IMINT and supplementary reporting. For example, in the same interview, Gen. Fazli claimed that 38,206 personnel in 164 battalions were participating in exercises across the country, which translates to 233 personnel/battalion.
Similarly, exercises held near Tehran in January 2014 featured a total of 1,140 personnel in four battalions, which translates to 285 personnel/bn, some of which can be assumed to be training staff and not members of the battalion themselves. Another exercise in south-western Iran, in the Ilam province, featured a battalion with 213 personnel.
Meanwhile, imagery from exercises in the northern province of Gilan shows approximately 250 personnel gathered together for review, which is well within the error-range when making estimates based on the number of rows and columns in a formation.
|Imam Hussein Bn, Gilan|
These battalions are reportedly composed of three rifle companies of 56 personnel each, which is consistent with open-source IMINT. Their designations are based on their ordinal position within the battalion (ex: first, second, third).
For example, during weapons-training at an unknown location, personnel were shown loosely clustered into companies with an average strength of 52. In Gilan, a company was shown at strength of 48-54, while in a gathering in Tehran the number was 57, and in an unknown location, a company of 51-57 was shown on parade.
|Rifle Co, Gilan|
This translates to a total combat strength of 168 personnel, leaving 66 battalion personnel unaccounted for. These can likely be found in the Basij equivalent of a Headquarters and Headquarters Company responsible for peace-time administration and battlefield support.
There is little evidence to suggest the existence of a battalion-level weapons company or platoon, though some of this responsibility may be subsumed under the 66 personnel noted above. There is a low-to-moderate probability of a DshK-equipped machine-gun or SPG-9-equipped recoilless-rifle section organized at an unknown level and strength.
|Active-duty IRGC-GF troops providing weapon-familiarization |
training on the SPG-9 recoilless-rifle
Works Cited / Footnotes:
 http://www.yazdfarda.com/news/1392/11/83657.html http://gordan-basij.blogfa.com/post/1