The Lord Commander Militant is the supreme commander of all Astra Militarum forces in the Imperium of Man, the highest-ranking individual in the Astra Militarum, and the representative of the Astra Militarum on the Senatorum Imperialis. However, the rank is primarily a political position, and the Lord Commander Militant spends the vast majority of his time overseeing the bureaucracy of the Departmento Munitorum and engaging in political in-fighting with the other High Lords of Terra. The actual administration of military matters and Imperial Guard strategy has remained in the hands of the Lord Commander Militant's immediate subordinates, the five Lord Commanders of each of the Segmentae Majorum.
Heth was the Lord Commander Militant of the Astra Militarum in the middle of the 32nd Millennium. The primary political rival on the Senatorum Imperialis to Lord High AdmiralLansung, Heth differed from the Admiral due to his desire to lead his troops on the battlefield directly and the suprisingly warm relationship he had maintained with the rank-and-file troopers of the Imperial Guard despite his rise to such a high position. Heth was joined in his opposition to Lansung by Inquisitorial RepresentativeMarguerethe Wienand and Grand Master of AssassinsDrakan Vangorich. During the War of the Beast, Heth personally led an Imperial Guard expeditionary force to relieve the Imperial Fists Chapter on Ardamantua from a massive Orkforce. However, Heth's forces were destroyed by the unexpectedly large and powerful Greenskin force, and before his voidship was seemingly destroyed the Lord Commander Militant despatched a message back to Terra warning of the coming of The Beast and the existence of his Attack Moon.
Abel Verreault was the Lord Commander Militant of the Astra Militarum the the mid-32nd Millennium who replaced Lord Commander Militant Heth after his unexpected death during the War of the Beast. Verreault discovered that most other Imperial forces were bogged down in executing other campaigns and what trrops were at his disposal lacked transport to get them in-theatre due to the machinations of Lord High AdmiralLansung. To gain this critical transport, Verreault ultimately agreed to follow Lansung's political lead on the Senatorum Imperialis in return for getting his Guardsmen into the fight. Unlike Heth, who had been a soldier's general, Verreault was much more of a politician. He supported the Proletarian Crusade and was always ready to support the wider political schemes of Lord Commander of the ImperiumUdin Macht Udo, the leader of the High Lords of Terra during this period. But when the Inquisition moved to remove Udo, Verreault saw which was the wind was blowing and quickly withdrew his support. Verreault was later assassinated by the Master of the Administratum Tobris Ekharth as part of the plot by Drakan Vangorich, the Grand Master of Assassins, to take over control of the Imperium during the attempted coup remembered as The Beheading.
After the assassination of Abel Verreault during The Beheading, Oskar Lowis replaced Verreault as the new Lord Commander Militant of the Astra Militarum on the Senatorus Imperialis. However, he was only a political puppet of the Grand Master of AssassinsDrakan Vangorich, and carried out Vangorich's plans for the Imperium until the Grand Master was slain by the intervention of the Adeptus Astartes, which selected a new Senatorum Imperialis.