This is the history of the Ultramarines, one of the Chapters of Space Marine in the Warhammer 40,000 Universe.
The Primarchs
The Emperor created the 20 Primarchs to aid him in his fight against the enemies of humanity, each was a giant of a man, more God like than human. The Gods of Chaos learned what was planned and spirited away the tiny infants though space and time, many taking years to arrive at their resting place. Many say that during this time damage was done, and this was responsible for the terrible events that occurred during the Horus Heresy. None can say for sure if this is the case. These is the event of one of those infants - Roboute Guilliman.
The capsule containing the young Primarch drifted through space for many years before settling on the world of Macragge which was a bleak industrial world which had been inhabited by man for many centuries. His capsule was found intact by a group of noblemen who were hunting in the forest. They broke the seal of the capsule to reveal a child surrounded by a glowing nimbus of power. The infant was taken before the Consul - Konor - who adopted the child as his own, naming him Roboute.
As with all the Primarchs, Roboute grew exceptionally fast, learning and adapting to his environment as he did. By the age of ten he had learned all that he could from the wise men and the elders. He had a deep insight into matters of history, philosophy and science, however his greatest talent lay in war.
The young Roboute was given command of an expeditionary force in the North of Macragge - Illyrium. This area had long contained bandits and brigands, and although many campaigns had been fought against them, they had never been defeated. Roboute fought a brilliant campaign that not only won the battle, but the respect of his foes.
Upon his return home, he discovered the city in turmoil. A conspiracy, led by a wealthy merchant named Gallan had seized control and overthrown his father. Roboute found his father dying of wounds inflicted by a hired assassin. He quickly crushed the rebels and restored peace and order to Macragge.
When the Emperor reached Macragge, Roboute had been ruling for five years. During this time the planet had undergone a massive change - the people were well fed, the armies were strong and well trained and regular trading routes had been set up with local systems. The Emperor realised that Roboute was a man of great vision and ability.
The Ultramarines
Due to his ability to plan and organise, the Emperor gave command of the Ultramarines Legion to Roboute Guilliman and the main base was relocated to Macragge. It took the Primarch little time to assimilate the wonders of the Imperium and he quickly led the Ultramarines to various victories in the galactic South. He liberated more worlds during the Great Crusade than any other Primarch, those that were freed benefited from his organizational skills. He set up a self supporting defence system, along with advisors to aid with trading and government. As this was occurring the Marines left behind on Macragge supervised the work over the Fortress, and also a training base which was producing fresh recruits within a year. By the end of the Great Crusade the Ultramarines were the largest Legion, they also took the fewest casualties due to their Primarchs tactical expertise.
During the Horus Heresy, the Ultramarines were poorly placed to aid in the battle. They were fighting on the Southern arm of the Galaxy and didn't receive word of the battle until it was too late. Their main contribution was the destruction of a large force of Chaos Marines which were on their way to reinforce Horus. After the fighting was over and Horus was defeated, the Ultramarines found themselves scattered all over the galaxy. Always the largest Legion, they had been largely untouched by the whole affair, and they were relied on to hold the Imperium together as it reeled from the blow struck by Horus. The Ultramarine base was able to supply recruits at such a rate that Ultramarines accounted for over half of the total Marines in the Imperium. However, a new theory of warfare and tactics was emerging as the Codex Astartes took shape, under the direction of Roboute Guilliman. This would lay the foundations for all of the military strength of the Imperium of the future.
The Second Founding
The Second Founding occurred seven years after the death of Horus. Whereas the old Legions were of an unlimited size, the new Chapters were nominated at 1000 warriors. As each Legion was dissolved down, one chapter kept the name and livery of the old Legion, while the rest took new identities. Some only split once or twice, such as the Space Wolves who only split into two Chapters. However, the Ultramarines were so large that they split many times, the exact number has been lost, one guess being twenty three. The new Chapters created from the Ultramarines gene seed are often referred to as 'Primogenitors' or 'First Born'. All of these Chapters venerate Roboute Guilliman as their own founding father and Primarch.
The military of Ultramar
Ultramar is the realm of the Ultramarines within the greater realm of humanity that is the Imperium. Its worlds do not pay the Imperium's tithes that other worlds do. Instead they contribute directly towards the upkeep of the Ultramarines Chapter. The rulers of the individual worlds of Ultramar are feudal lords of the Master of the Ultramarines. This is why the Ultramarines' leader is also known as the Lord of Ultramar. Just like other worlds in the Imperium, each world of Ultramar raises regiments for its own defence. Most worlds in the Imperium are obliged to provide regiments for the Imperial Guard when required, but Space Marine home worlds are an exception. In the case of Ultramar however, the Ultramarines rule so efficiently and are so prosperous that they maintain several hundred regiments ready to join the Imperial Guard when needed. As a result regiments have fought all over the galaxy, often in campaigns alongside the Ultramarines themselves. Each world of Ultramar also provides Space Marine recruits for the Ultramarines Chapter itself. Throughout Ultramar proud citizens point to public statues of famous Ultramarines who were born to local families. Amongst the older aristocratic dynasties it is a matter of considerable esteem to provide recruits for the Space Marines. For a family to have provided a renowned hero, or an actual Master of the Chapter is a great honour bringing considerable fame for generations.