Okay so here's my pic for that art contest thing and here the explanation of each trooper from left to right:

Each of these factions troops represent a government in the Allies

ERS: The Black armored trooper is a the ERS (Earth Republic of Systems) commando. The military is split up into four branches where the Marines are the expeditionary force of them, the Army functions as a colonila force: recruitment comes from the planet it's stationed on, the Navy acts as everything from system patrol, to military transport and Section 6: the brainchild of the military and spooks of the government. The ERS is a continum of modern day Democracy after the The Purge and Intersteller Migration. It operates like American colonialism before WWI

RA: The Red Army is the backbone of the military of the Red Republic. Like the name suggests its the child of modern-day Russia and a alliance with old communist Russia states. Each citizen of the RA is required to serve for a least four years in the military giving the Red Republic the largest army in the galaxy.

EUE: The Economic Union of Europe. European countries after the Instersteller Migration were divided until the first Galactic Depression. An Irish colony discovered a faster way to process anti-matter. With it's discovery ships now had a new efficent fuel that gave the EUE it's start and jump to the #1 economy. The power armor of the EUE infantry is based off old knight armors from the past.

OA: Oriental Alliance. The OA operates with a different sort of goverenment with a oligarthy of the top buisnesses of the Empire. The OA were the first to discover the potenial of Power Armor, Pulse Weapons and Plasma technology (shielding, cannons, etc). The troops of the OA are taught in the way of the Samuria and scouts are taught the way of the ninja. OA is the top R&D developer and a vital source for the Allied military.

Any questions you have I'll answer