You are probably here because you asked how to start a Space Marine Army. To answer that question you need to be able to answer a few questions:

• Do you want to play Warhammer: The Horus Heresy or Warhammer 40k?
• Do you want to play a Astartes/loyalist or a Chaos/traitor legion or chapter?
• If your answer are 40k and loyalist then do you want to use ‘regular/first born’ Space Marines or the latest and greatest Primaris Space Marines?

If you cant answer or have no idea what the above refers to then read the lore summary below to get a quick grasp so might be able to answer the above questions and the community can better help you.

Lore overview

Birth of space marines

In the late 30th Millennium (30k, 792.M30), the Emperor of Mankind still walked amongst mortals. In that time, the Emperor first created the Primarchs, 20 immortal superhumans blessed with extraordinary intelligence, charisma and sheer physical might. The Primarchs wielded powers the like of which are not known in the Imperium of the current timeline progression (40/41st Millennium currently lore wise - hence the name 40k), yet they were lost to the Warp in an accident, still in their gestation capsules, scattered through the Immaterium to worlds across the galaxy by the will of the Dark Gods of Chaos.

The first Space Marines were also the creation of that 30k era, each made using the genetic inheritance of the Primarchs (gene-seeds), and the arcane alchemies the emperor developed allowed generation after generation of Space Marines to be created for the Imperium.

The gene-seed implants are 19 specially grown organ grafts. Some serve to give the Space Marine unique abilities, such as the Betcher’s Gland, which allows them to spit poison. Others enhance existing abilities, such as the Sinew Coils, which dramatically increase strength and durability. Implantation of these organs must occur in a precise order.

The Space Marines were originally divided into 20 large Legions created during the First Founding by the Emperor, and each Legion was filled with thousands of Space Marines, separated based on whose gene-seed from each from one of the original Primarchs was used.

When 18 of the 20 original primarchs were rediscovered during the Great Crusade, they became the commanders of whichever was Legion was genetically related to them.

The Horus Heresy

During the Horus Heresy (002.M31 onwards, approximately 210 years after the creation of the Primarchs), half of the Primarchs and their Legions turned Traitor to the Imperium and swore themselves to the Ruinous Powers of Chaos, becoming the 9 Traitor Legions of Chaos Space Marines. The war they waged against the Emperor/Terra for 7 years is called the Horus Heresy, named after the Primarch of the Luna Wolves Legion (renamed Sons of Horus) who lead the rebellion, Horus Lupercal. This is the setting for Warhammer, The Horus Heresy game, often called 30k.

The Horus Heresy devastated the Imperiurn. The damage wrought by the Traitor Legions was so severe that the Emperor’s realm never fully recovered. The terrifying power of the Space Marines was shown more clearly than ever before, and, their fallibility tragically revealed. It was clear to Roboute Guilliman, Primarch of the loyalist Ultramarines, (blue poster boys of space marines) that the Primarchs held too much power, that the awesome might of the Legions was simply too dangerous to place under the control of so few individuals. Thus he composed the Codex Astartes. Into it he poured every iota of his deep military and logistical knowledge. No detail in all of the Codex’s many thousands of pages was more significant than the decree that the awesome fighting forces of each Space Marine Legion - which at their height included many tens, or even hundreds of thousands of warriors - be broken down into much smaller brotherhoods called Chapters with a maximum of 1000 space marines. Each of these fighting forces would be independent, with complete autonomy over their actions and recruitment, and have their own heraldry and colours.

Warhammer 40k

Lore and Warhammer 40k game wise it is now the 41st Millennium and it is believed that there are over a thousand Space Marine Chapters fighting the Emperor’s wars. Nine are named for the original Legions that remained loyal to the Emperor during the Horus Heresy.

Over the millennia, the number of Space Marines has waxed and waned. Some have been destroyed, wiped out in cataclysmic battles with the fiercest of enemies, but their existence has been deemed so vital to Humanity’s defences that there have believed to have been 27 Foundings (orders for creation of space marines) throughout the millennia. The very first Founding was of course the creation of the original twenty Space Marine Legions by the Emperor himself.

Primaris Space Marines

The Ultima Founding is the most recent lore wise. Consisting entirely of Primaris Space Marines, it was created at Roboute Guilliman’s direct order shortly after his resurrection (after close to 10000 years in stasis since Horus Heresy). It is arguably the largest and most significant Founding to date.
In addition to the nineteen specialised organs that almost all Space Marines have been implanted with since the First Founding, the Primaris Space Marines possess a further three. These organs enhance their physiology to an even greater degree, making them even stronger and tougher. It was only thanks to the Sangprimus Portum - a device that held potent genetic material harvested from the Primarchs themselves - that Archmagos Belisarius Cawl was able to create these biotechnical miracles. In the Ultima Founding, many tens of thousands of Primaris Space Marines reinforced depleted Chapters or formed new ones. Since then, many Chapters have used these technologies to develop their recruits into Primaris Space Marines. The biggest breakthroughs of all, however, were the surgeries that allowed original Space Marines not matured with the three Primaris organs to be implanted with them and to grow even further into Primaris Space marines.

Community Note: OG space marines are commonly referred to as first born space marines. The models are shorter than Primaris space marines. Furthermore, GW miniatures are in what’s known as “heroic scale”, which accentuates bodily proportions in certain ways to make them look better/less weird than typical human proportions, especially noticeable with hands, heads, etc. The generic first born space marines models aren’t accurate to the descriptions of them in the lore; too short and are missing their abdomen (rib cage essentially sits on hips for the models). People have taken to converting/modifying first born models to make them more representative of the law and proportions and they are called true-scale.

Community Note: There is a disdain for the Primaris models by some of the community for a couple reasons:

Lore wise - They are a tacked on “solution to a problem that doesn’t need a solution”. Story wise there has just been the return of Guilliman, a character that is sort of disliked by many players for being the “golden child”, and in an age of human technological regression, there is a newly introduced Deus Ex Mechanicum character of Cawl, who has “improved” the emperors design (humans haven’t even figured out how the emperor did it in the first place for 10000 years and now someone is doing it BETTER), AND he invented new weapons!?

GW Greed - GW wanted to make bigger Space marines, but they knew that people wouldn’t take kindly to them just replacing the old models. The current “first born marine” line is only 5 years old, so replacing it would probably not be a good idea. Secondly they “need” to constantly update Space Marines, to sell more… So two problems that needs addressing. GWs solution? Make up a story about bigger and better Marines that is full of holes and is obviously just about making money.

While GW has said that First Born models will continue to be produced/sold/supported a lot of the community still doesn’t believe it, especially after the complete discontinuation of Warhammer Fantasy and the creation of Age of Sigmar in its place.

Decisions

Now that you have a better understanding of basic Space Marine law you can make some decisions. If you are interested in playing loyalist astartes have a look here at some of the factions