The Great Crusade
FoundingTerra, where the they earned their first battle honour, 'Roma.' The Legion had the benefit of being united with their Primarch shortly after their formation, while their character had yet to take shape and as much as 70% of their ranks were still aspirants. As result, the Imperial Fists and Rogal Dorn developed as servants of the Imperium together, forming an unbreakable bond based upon the common desires for self-discipline and total commitment to order found in Dorn's temperament and the gene-seed of the Legion.2a
Doctrine and RecordThe general disposition of the Imperial Fists was to embrace their military role in the Great Crusade and eschew the civil responsibilities of governing worlds for recruitment and other purposes. For example, after being granted recruitment rights on Necromunda following a major victory against the Orks, the Legion situated itself as guests rather than masters. On the subject of the Legion's relationship with recruitment worlds, Dorn is famously recorded as saying "I want recruits not vassals."2a Such an attitude was in contrasts to other Primarchs, such as Perturabo of the Iron Warriors, who took every opportunity to garrison worlds, claim their tithes and develop a personal empire.
In prosecuting the Great Crusade, the Imperial Fists acted as the strategic reserve of the Emperor's forces and served as his personal Praetorians. As a strategic reserve force, the Legion rapidly deployed between battlefields, characteristically employing siege and defensive warfare as the situation required, specialising in each. As siege specialists, the Legion functioned as an assault formation, surgically applying force where and when it was required to shatter enemy defenses, often deciding the outcome of campaigns. As defensive specialists, the Legion was commonly tasked to defend strategic positions and defeat enemy breakthroughs.2a 2d In fact, Horus once remarked to Dorn that "if I ever laid assault to a bastion possessed by you... then the war would last for all eternity, the best in attack matched by the best in defence."10 The Legion also possessed special skills in urban warfare. 2a
The Imperial Fists were so effective in their role during the Great Crusade that the Legion accumulated a record second only to that of the Luna Wolves.10 As a proud testament to their accomplishments, battle honours were displayed for kilometres in a gallery aboard the Phalanx.7 however, the Imperial Fists' record also included developing hostile relationships with other Legions. Most notably, the Imperial Fists maintained a bitter rivalry with the Iron Warriors, which was largely the result of both Legions sharing similar combat specializations.30 Less known was the strong mutual hatred between the Imperial Fists and Alpha Legion, who clashed on more than one occasion prior to the Horus Heresy, though the nature of these clashes is unknown.31
After the successful conclusion of the Ullanor Crusade, the Emperor returned to Terra, ordering the Imperial Fists with him where they were tasked to fortify the Imperial Palace.10
The above is taken directly from the Warhammer 40k lexicanum.com I have not written it but I find it to be a great source for all thing 40K info wise so I wanted to share it.