The Imperialis Militia, also known as the Imperial Militia, was a large auxiliary and reserve branch of the Imperial Army during the Great Crusade and Horus Heresy. The Imperialis Militia was analogous to the Planetary Defence Forces of the 41st Millennia.
The first units of the Imperialis Militia were formed during the Unification Wars by the Emperor, where they comprised of normal Human auxiliary units who fought alongside the early Imperial Army and Thunder Warriors. By the time of the Great Crusade, the Imperialis Militia was tasked with occupying and garrisoning worlds brought into the early Imperium. Militia units would be deployed on newly pacified worlds as the Expeditionary Fleets of the Great Crusade moved on to their next conquest.
Much like the PDFs of M41, the Imperialis Militia was not a galaxy-spanning military force. Rather, its makeup varied greatly world to world and was led by a variety of local Imperial Commanders, Officers, administrators, and even tribal kings. However it was very common for Militia units to consist of a core of professional soldiers which in times of war would be greatly augmented by a force of reserves and conscripts. Also similar to the PDFs of the 41st Millennium, the Militia tended to be a more lightly equipped and poorly trained force when compared to regular Army units, Solar Auxilia, or the Space Marines.
The Imperialis Militia used equipment very similar to the Imperial Guard of the 41st Millennium. Individual armaments were highly unstandardized and often locally provided, resulting in infantry going into battle with anything from Lasrifles, Lascarbines, and Bolters to black powder muskets and crossbows. Armored vehicles included Leman Russ Battle Tanks, Gorgon Armoured Assault Transports, Sentinels, Malcador Heavy Tanks, Basilisks, and Medusa Siege Tanks. They also operated Super-Heavy Tanks including Baneblades and Stormhammers. Air units often consisted of Thunderbolts and Arvus Lighters.