The Imperial Guard is the largest fighting force of the Imperium of Man. It is comprised of countless billions of men and women -- hundreds of thousands of different regiments, supported by a vast array of light and heavy armoured vehicles that provide the Guard's primary offensive punch. They are usually the first Imperial force to respond to a threat if a planet's Planetary Defence Force (PDF) fails to suppress it. They also garrison major locations of strategic or cultural interest to the Imperium and are often found in defensive roles. The Imperial Guard is often referred to as the "Sledgehammer of the Emperor"; the sheer amount of force that the Imperial Guard can bring to bear on the enemy is devastating, but is not as direct or as precise as their Space Marine allies, who are described as the "Scalpel of the Emperor."
The Imperial Guard 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 Imperial Guard is not a swift and precise military tool like the Space Marines. 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 Astartes, there are thousands of men in each regiment and it is in the sheer size of its forces that the true strength of the Imperial Guard lies.
In large-scale Imperial Crusades that usually invole large armies of the Imperial Guard, Guardsmen High Commanders are often found directing the Crusade effort from the rear, keeping with him representatives from the various Imperial Guard Regiments and the occasional Space Marine Chapter involved in the Crusade to keep in constant contact with the fighting components. However, when necessary, the High Commander may take to the battlefield with his aides and bodyguard, boosting his men's morale with his mere presence. High Commanders have a higher rank than other officers normally found at the front lines. A commander has the choice of which regiment to lead from the front, but they can be put into the major sections of all different types of Imperial Guard Regiments.