Short answers, pulled from the story.
The Choctaw Nation sided with the Confederacy because they believed the United States was on the verge of collapse, felt neglected by the federal government, and their laws supported slavery. William H. Seward advocated for seizing Indian lands while their main agent supported the South.
By June 1861 the Choctaw Nation had declared itself free and independent and appointed commissioners to make an alliance with the Confederacy. This decision followed a period where officials in Washington City assured Unionists that the nation would remain neutral.
Confederate envoy Albert Pike negotiated the Treaty with Choctaws and Chickasaws conducted in July 1861. The treaty covered sixty-four terms including sovereignty issues and Confederate citizenship possibilities for the nations.
The First Choctaw and Chickasaw Mounted Rifles were tardy and missed the opportunity to engage at the Battle of Pea Ridge. A disgusted officer later acknowledged that no tents, clothing, or camp equippage was furnished to any of them except for partial supplies for the Choctaw regiment.
A total of 101 men were mustered into the 1st Choctaw Battalion in March 1863 under the command of Major John W. Pierce. The battalion formed in February and disbanded in May 1863 before members transferred to Spann's command in late August 1863.
On the 19th of June 1865 Principal Chief Peter P. Pitchlynn surrendered the military at Doaksville in the Choctaw Nation. The Southern Treaty Commission crafted the 1866 Treaty with the Choctaw and Chickasaw which included provisions for slavery abolishment and amnesty for siding with Confederate States of America.