In the Warhammer 40,000 universe, the term "Legion Imperialis" refers to the Imperial Army, the vast military force of the Imperium of Man. The Imperial Army was composed of countless regiments, soldiers, and war machines, serving as the primary ground force for the Imperium in its countless campaigns and wars across the galaxy. During the Great Crusade era, before the Horus Heresy, the Imperial Army consisted of numerous expeditionary fleets, each led by a Primarch and their Space Marine Legion. These forces worked together to conquer and reclaim human-settled worlds in the name of the Emperor of Mankind. After the catastrophic events of the Horus Heresy, where the Space Marine Legions were divided and turned against each other, the Imperial Army was reorganized. It underwent significant changes, eventually leading to the establishment of the Imperial Guard, which is the mainstay of the Imperium's ground forces in the grim darkness of the 41st millennium. The Imperial Guard, also known as the Astra Militarum, is composed of countless regiments from various planets, each with its own distinct culture, equipment, and fighting style. The troops of the Imperial Guard are considered the foot soldiers of the Imperium, facing a myriad of threats ranging from xenos invasions to Chaos incursions, always in the service of the God-Emperor. The Legion Imperialis, as part of the broader Imperial Army, embodies the vast, diverse, and often unyielding military might of the Imperium of Man in the grim and brutal universe of Warhammer 40,000.