American Indians had as varied reactions to the outbreak of war as white Americans did, with direct participation in the war, attempts to take advantage of the conflict, and outright ignoring it.

You've asked a question worthy of a book-length answer. There is no one "American Indian experience" of the American Civil War any more than there is a universal "white" or "black" experience. Tribes and individuals reacted differently based on different factors. Some tribes and individuals sought to participate in the war, actively enlisting in one side or another. Others sought to take advantage of the situation in one way or another, either by revolting against the federal government or exploiting the withdrawal of regular soldiers from forts in the West. Still others simply ignored the conflict entirely.

I recommend Civil War in the American West as one of the better introductions to the topic, both for the scholarship it spawned and because it's a good book in and of itself. A good example of the type of work it spawned is The Prairie Was On Fire: Eyewitness Accounts of the Civil War in the Indian Territory, published in 2001 by the Oklahoma Historical Society and an engrossing read if you can find a copy.

If you want something you can look at immediately, without spending money, professor Annie Abel of Smith College drafted a three-volume history of American Indians in the years leading up to the war, the war itself, and during Reconstruction. That work is more than a century old now, however, and its approach to the topic is more than a bit out of date. Even American West, now more than two decades old, is showing its age, but it's still a good introduction.

In the West, at the outbreak of the war, there was a divide between the Indians who lived in Kansas and Oklahoma. Many who lived in Kansas had been settled there from removals in northern states, and they tended to have northern attitudes. In Oklahoma, or Indian Territory as it was then known, the Five Civilized Tribes (Creek, Cherokee, Choctaw, Chickasaw and Seminole) were distinctly southern in attitude. The richest among them owned plantations and kept slaves, even after their fellow slaveholders drove them from their original homelands in Florida, Mississippi, Alabama and Georgia.

In February 1861, the Choctaw tribal council passed a series of resolutions indicating that their nation was prepared to join the Confederacy. They had been promised restitution for their lost tribal land, but Congress was slow to appropriate the money, and bad feelings abounded in Doaksville, the tribal capital.

Within other tribes, there was division just as there was across the United States. The Cherokee, for example, would fight a civil war within the civil war. In May 1861, Chief John Ross would issue a neutrality proclamation saying the tribe would be neutral in the war due to its obligations to the United States. At the same time, Stand Watie, Ross' political rival and the man who would become the most famous Indian in the war, was already forming a mounted rifle regiment for the Confederacy. In July, he was promoted to colonel and sent to Arkansas.

In early August 1861, Union forces were defeated at Bull Run and a small battle at Wilson Creek, Missouri. Following these southern victories, a saavy Confederate recruiter named Albert Pike visited the Five Civilized Tribes and convinced them to join the confederacy. He promised that the Confederacy would honor their sovereignty and resume the payments that the federal government had stopped at the outbreak of the war.

With Pike's political maneuvers, Ross publicly reversed his position, and the Cherokee threw their support behind the Confederacy. Other tribes did too, but not all.

Opothle Yahola was a wealthy landowner and Upper Creek man who organized opposition to the Confederate tide. He had lived through the Creeks' removal from Alabama, and he was reluctant to participate in what he felt would be another losing war.

Many Indians who felt as he did gathered at Yahola's plantation. When a Confederate party demanded he sign a treaty with the Confederacy, Yahola instead fled north with his likeminded compatriots ─ all 9,000 of them.

They were bound for Fort Row in Kansas, 250 miles north, and it wasn't an easy trip. They avoided two attacks from Confederate guerrillas, but they weren't lucky the third time. On the day after Christmas, 1861, Yahola's party was attacked at Chustenahlah. Many of the party dropped their supplies and ran for their lives. By the time they reached Fort Row a few weeks later, about 2,000 of them had died from exposure, malnutrition and combat.

Those who lived were itching for revenge. Many signed up for the Indian Home Guard, created in Kansas, and others joined the First Kansas Volunteer Regiment, which mostly included escaped slaves. They were joined by others from Oklahoma who hadn't joined the Confederacy. The Delaware, for example, almost entirely went for the Union. At places like Pea Ridge, Cabin Creek and Honey Springs, the Kansas men and others from the North repeatedly defeated the Confederate forces, driving them back again and again.

With the Confederate defeats mounting, John Ross mounted opposition to the Confederate treaty, but he was stuck in a no-win situation. At the Battle of Pea Ridge, northern soldiers had reported (correctly or not) scalping of injured Union men by the Cherokee soldiers, and compromise became much less likely. Stand Watie fought to the end, becoming a brigadier general in the Confederacy and surrendering on June 23, 1865, long after the events of Appomattox.

Now, this only addresses the fighting in Kansas, Missouri and Oklahoma. There was plenty else happening, so read on ...