Astra Militarum - Order of Battle: Each Astra Militarum regiment is a powerful combat force, but it is only when the constituent infantry platoons and tank squadrons are fielded together on the battlefield that the deadly potency of the Astra Militarum is truly realised. Lasguns, Battle Cannons and devastating ordnance fire in coordinated salvoes to eradicate the enemies of Humanity.The organisational structures of the Astra Militarum are based upon the Tactica Imperium, which groups squads together based on their categorisation. Infantry platoons are pooled together in infantry companies, which then make up infantry regiments, and similarly tank squadrons are grouped into tank regiments comprised of tank companies. However, the many varied demands and challenges of war usually mean that these structural theories cannot be applied on the battlefield. Instead, the various constituent parts of each regiment are often reorganised into much more varied and versatile deployments.The war zones of the Imperium are as diverse as they are numerous. Some are grinding battles of brute force and attrition, whereas others require guileful feints and swift redeployments. Where one war zone may call for a mechanised advance over a barren planetscape, another will require infiltration into a Heretic-infested hive city. The multifaceted nature of the Astra Militarum allows battle groups to be effectively deployed against all the multifarious enemies of humanity. The separate components of a Militarum Regimentum are drawn together to form a cohesive fighting force, and if an operational commander can successfully navigate the bureaucracy of the Departmento Munitorum, these battle groups can be tailored to the specific enemies and environments they must face.The Astra Militarum is the largest and most diverse military organisation that the galaxy has ever known. Numbering in the billions and equipped with innumerable tanks and super-heavy war machines, the Imperial Guard serves as both the hammer and the anvil of the Imperium of Man. The Astra Militarum is not a swift and precise military assault tool like the Space Marines of the Adeptus Astartes. Instead, it is used to bludgeon a foe into oblivion in bloody wars of attrition that can last for Terran months, years or even decades in many cases. Although a single Imperial Guardsman can never be the equal of even the most raw and unblooded transhuman Astartes, there are thousands of men and women in each regiment and it is in the sheer size of its forces that the true strength of the Astra Militarum lies.An Astra Militarum regiment is mostly uniform in its force composition. Infantry regiments contain no heavy artillery, whilst armoured regiments contain no infantrymen. As a result of this policy, regiments are required to work together in combined arms formations known as battle groups or task forces. This order of battle dates back to the days immediately following the end of the Horus Heresy in the early 31st Millennium, when the High Lords of Terra decreed that Imperial Guard regiments should be separate yet interdependent. This was done so that no faction of the Imperial Guard would ever be able to rebel against the Imperium with such devastating effect as the combined arms regiments of the Imperial Army who had joined the forces of Chaos and supported the traitorous Warmaster Horus.The Tactica Imperium sets down the basic template around which all Astra Militarum regiments are to be organised. Each is divided into companies, some with as few as 3, others with as many as 20. Companies are further divided into between 3 and 6 platoons and these platoons each consist of between 2 and 6 10-man squads led by a command squad whose commanding officer is a lieutenant. Terms and naming conventions may vary from regiment to regiment, and while the regimental commanding officer is normally accorded the rank and title of colonel, the military and planetary culture from which they are drawn may use a different title for the same rank, ranging from "Knight Commander" to "Chief Hetman," whereas the commander of a company could have the title of "captain," "Taxiarchos" or "Marzban."This structure is maintained in all the different types of Astra Militarum regiments. In an armoured regiment the individual guardsmen form tank crews, with each tank taking the place of an infantry squad as the basic unit. These tanks are then formed into squadrons, led by an officer's command tank, and further organised into armoured companies, a number of which make up the entire armoured regiment. In an armoured regiment that consists of super-heavy tanks, each individual tank is treated as being equivalent to an infantry platoon, meaning that such an elite regiment will only number 9 to 12 super-heavy tanks, in addition to its numerous and uncounted supply and services vehicles.When an "all-arms" force is required, units are drafted from the available regiments and placed under the command of a senior officer, such as a colonel or a captain. These "battle groups" can vary in size, from units of a hundred or so soldiers and five or six armoured vehicles, to a force almost as large as a single regiment. Some are a balanced mix of infantry and vehicles. Many, however, are highly specialised, designed to meet very specific tactical challenges. A battle group might consist of Chimera-mounted infantry and self-propelled artillery assigned to support a lightning-fast armoured breakthrough, or it may be an entire Sentinel company aiding a light infantry force in a jungle fight. Other battle groups are even more specialised, drawing together multiple tank and artillery squadrons to form an entirely armoured force. A "standard" battle group might comprise multiple infantry companies supplemented by several armoured, artillery and support companies. These can then be fielded in multiple detachments, each of which can have support personnel and materiel seconded from one of the other branches of the Departmento Munitorum, such as the Militarum Auxilla or the Officio Prefectus.On occasion, a lighter Astra Militarum force may be called for. In such cases, individual detachments are deployed. These small-scale forces adhere to the same hierarchy of command as the larger battle groups, with a Platoon Commander, Tank Commander or possibly even a Lord Commissar operating as the ranking officer. Similarly, these detachments can contain infantry, tanks, artillery or a combination thereof. A single detachment can be outfitted to engage a range of enemies on the battlefield, or it can be equipped to deal with a single, specific set of targets.Just as battle groups are comprised of several detachments, so too can several battle groups be drawn together into a huge, operational-level force known as an "army group." These forces are led by high-ranking commanders such as generals and high marshals and will typically contain battle groups from multiple Militarum Regimentos -- sometimes as few as two to as many as several hundred in the largest, sector-wide Imperial campaigns. The method by which Astra Militarum forces make it to a new war zone are often haphazard, with many thousands of troops from dozens of different regiments sent to a stellar region or star system under assault once the Imperial bureaucracy raises the alarm. Such are the vagaries of travel through the Warp, to say nothing of the difficulties of communication posed by the sheer size of the Imperium, that it is only because so many regiments are despatched that a sufficient number of forces arrives in time to be of any use at all in a specific campaign.It is quite common for thousands of Imperial Guardsmen to be lost in Warp storms during transit, to be attacked by Human or xenos pirates or diverted to an entirely different conflict as the need arises. Meanwhile, the high commander overseeing the battlefield can rarely craft a strategy before knowing which resources they will receive, and will instead have to improvise a battle plan based upon the troops at their disposal. That this usually proves more possible than not is a testament to the versatility of Imperial combat doctrine, but even more weight should be given to the sheer amounts of manpower and materiel that the Astra Militarum can bring to bear. With enough perseverance, even an improper tool can be used to reap results -- particularly if the commander is unconcerned with how many such tools are destroyed in the process.Once an Astra Militarum army group has been assembled where it needs to be, it operates on the same basic principles as a battle group, but on a far larger and more complex scale. Entire regiments of tanks advance under the covering fire of 100 Basilisk self-propelled artillery pieces, supported by human waves of infantry. While an Astra Militarum battle group is capable of taking a single tactical objective, an army group can be used to conquer and hold an entire world or even star system, and few enemies of the Imperium are capable of mustering a force that can withstand its sheer momentum.