After the war between the colonies and Earth ended with the Earth Sphere being unified under the pacifist ESUN government, numerous soldiers who had fought in the war were left with no place to go now that the fighting was over. Declaring herself the successor to her father, Mariemaia claimed she would fulfill Treize's ideals and soon attracted many former OZ soldiers who were loyal to Treize to her cause, along with other former soldiers who had nowhere else to go. However, Mariemaia was actually little more than a puppet for her grandfather Dekim Barton, who sought to use the Mariemaia Army and the ideals of Treize as a cover for his true agenda to simply establish the Barton family as the rulers of Earth.
In late A.C. 196, the Mariemaia Army began its advancement on Earth. With little military forces remaining in the Earth Sphere after the disarmament by ESUN, the Mariemaia Army was able to easily swoop in and conquer most of the planet, kidnapping Vice-Foreign Minister of ESUN Relena Darlian in the process. However, the Mariemaia Army still faced opposition from the Preventers, a small group of armed forces left over that were allowed to remain armed so as to prevent war from ever breaking out again.
Though they had occupied the ESUN capital of Brussels, Austria, the tide turned against the Mariemaia Army when their own citizens that they had conquered began to revolt against their rule, right as the Gundam pilots were closing in on Mariemaia's bunker. Eventually, Mariemaia turned against her grandfather after being quite literally slapped to her senses by Relena and Dekim was executed by his own soldier, who realized they had been manipulated into betraying Treize's ideals. The Mariemaia Army was disbanded and peace was restored to the Earth Sphere once more.