The War of the Seven Nations
The creation of the Sioux Nation and it's fight for autonomy and independence from other regional powers began with increasing pressure from settlers from United States of New England (USNE), the Union of American States (UAS), and the Republic of Louisiana.
Open war broke out when a joint army from the UAS and Louisiana marched into territories held by various Sioux tribes and their plains allies.
The plan, as envisaged by the governments of the UAS and Louisiana was to split the territory up into two equal parts held by each.
This expansionist attitude angered the USNE which officially entered the war with its own designs on the territory of the plains states--having recently expanded by incorporating the states of Michigan and Wisconsin into its own territory and displacing the indiginous peoples who lived there.
The Independent Sovereignty of Cascadia quickly joined the war on the side of the Sioux at the urging of tribal members of Cascadia's Assembly.
The Empire of Australia, while not an official combatant, is known to have supplied the Sioux with weapons, horses, and other supplies through the convert actions of the Empire's spy organization, 1151.
Envoys from Cascadia were sent to both Canada and the USNE who helped the Sioux negotiate quick agreements with each of those nations to cease fighting and fix borders at mutually agreed boundaries. While not officially stated, it was believed that the Cascadian envoys threaten open war with both nations should they refuse.
With supplies of technologically advanced weaponry from Cascadia and the Empire it fell to the military leaders of the Sioux Nation, notably Red Cloud and Sitting Bull, to secure peace through the defeat of UAS and Louisiana's armies in the field.
After a series of defeats in which the Sioux used their prowess with horses and several deftly staged feints to outmaneuver the slower armies of their two enemies.
The final, decisive battle of the war took place near the confluence of the Missouri and Kickatus rivers. More than 10,000 wounded prisioners from the armies of the UAS and Louisiana were taken and after the fighting General Albert Sidney Johnston (UAS) and General Alexander Angevin (Louisiana) both unconditionally surrendered to Sitting Bull.
After this defeat, the governments of the UAS and Louisiana negotiated a settlement which set the borders of the Sioux Nation and formally recognized the territory as a soverign nation.