It's still a work in progress, but here is what I have now.
Merican 1st Expeditionary Unit
Size: 15,000 troops, armor support
Specialties: City fighting, trench warfare, Blitzkrieg
Homeworld: Terra (They are from the area of the former US)
They are a fleet based force in continual service.
Known Campaigns: Medusa V, 1st and 2nd Tyrannic Wars, 12th Black Crusade, Battle for Thanatos
Other Campaigns: Defence of Terra M32
Commander: General Anthony Jackson
Prefered Heavy Weapons: Autocannon, Lascannon
Prefered Infantry Heavy Weapon: Melta Gun, Grenade Launcher
Fleet: 2 Emperor Class Battleships crewed by men from Merica's Hives, supported by Merican Naval Fighter, and Tactical Wings.
Uniform: Tan-ochre fatigues, Brown Armor.
History
Great Crusade to the Heresy
When the Unifacation of Terra started, the unit sided with the Emperor, and fought alongside the 2nd Space Marine Regiment (old style SM organization). They then participated in the Great Crusade, and supported the 2nd Legion throughout the campaign. The legion even gave the unit a commander from their numbers. He stayed with the unit throughout the crusade. However, the Horus Heresy, where they defended Terra with everything they had, occured. They were at the Eternity Gate, when Terminators from the Iron Warriors slayed the valiant Space Marine commander, the last of the II Legion. Even in death, however, he held the regiments banner, the Stars and Stripes, off of the ground. When the unit saw this, they swarmed over to defend the body, and the banner. They fought off wave after wave of traitors, and finally, the siege ended. After the battle, the Space Marine was given a proper Merican Military burial, along with the dead of the unit.
The Scourging
After the death of their leader, the Merican Expeditionary Unit, fought with a new type of vigour. Just as their ancestors would fight valiantly after vicious attacks, so did they. During the Scourging, they fought alongside the Ultramarines on the many fronts in a now shattered Imperium of Man. Howver, they did not allow this dire time back them down. The memory of Merica burning from the traitor's guns was still fresh in their minds and they eliminated Traior warlords one after the other. In the Istvaan system, they lead many attacks against the rebels. On Istvaan III, they were issued hostile enviroment gear, and sent to cut down any resistance. They encountered a battle between what seemed to be untainted traitors, and warp "gifted" marines. The latter won, and set upon the Mericans. They were crushed. The bodies of the pure were investigated, and were found to be marines who had been betrayed. They had kept Imperial insignia, and on some bodies, torn, tattered and dog-eared copies of the Lectitio Divinitatus were found. They were circulated amongst the men, and added to a unit library. As the bodies were discovered, power armoured figures were seen to come out of the ruins around them, and attacked. As they fought, Space Marines in armour marked with an I symbol attacked the traitors and drove them off. They met the mysterius force's commander. They were informed that the dead loyalists were survivors of the opening moments of the Great Betrayal. They were also told that they had died a warror's death, fighting for the Emperor. The unit was told to bury them in the way that the Lectitio Divinitatus told them, with military honours as well. They kept their identification tags, and they were listed as Unit members that were KIA, their Legion names were reverted to original Pre-Primarch names, with the exception of Emperor's Children. They are as follows:
Brother Lieutenant Cassius Vernok
-Warhounds
-ID: 012121994
Brother Sergeant Gee Grilnok
-Dusk Raiders
-ID: 012121995
Brother Fabian Alexan
-Former Emperor's Children
-ID: 012121996
Brother Dragar Yade
-Luna Wolves
-ID: 012121997
Brother Hoyt Alcantar
-Luna Wolves
-ID: 012121998
Brother Michal Thoran
-Dusk Raiders
-ID: 012121999
Brother Johny Coxand
-Luna Wolves
-ID: 012122000