If the US Army's 10th Mountain Division did fragment or disband, it can safely be assumed that it occurred in the aftermath of the September attacks.
Officially, the Allied States Army's 10th Mountain Division was sent to Kansas to assist Jennings & Rall in their effort to rebuild and pacify the area and ensure it maintained loyalty to the Allied States of America. The ulterior reason for 10th Mountain Division elements to go specifically to Fillmore County and Jericho was to hunt down and capture Sarah Mason.
A detachment of the Allied States Army's 10th Mountain Division is headquartered in Jericho and is commanded by Major Edward Beck. At the end of Season 2, Major Beck, convinced of the complicity of the Allied States' goverment in perpetrating the September attacks on the country, announces that he'll no longer be taking orders from the Cheyenne Government. His inner-circle of Captains decide to join him later on. It is unknown what happened following this. It is speculated that the Division units directly under Beck's command may have defected to Texas or the United States of America.
- The 10th Mountain Division also exists in real life. The Division's insignia is almost identical to the US Army's 10th Mountain Division insigna, except for the addition of a gold star above the crossed bayonets and the transposing of the patch's basic colors (the US Army's insignia has red bayonets on a blue background with a blue "Mountain" tab). The significance of the gold star is not known.
- The reason why the insigna colors were changed is unknown. Jennings & Rall may have chosen to change the unit's insigna to distinguish it from the United States 10th Mountain Division if it exists