The Unification War was a war between the New United States and an alliance of seceded states known as the Free States. At the end of 2069, the newly elected NUSA President, Rosalind Meyers, presented a unification program to extend federal rule over the rogue Free States under the pretense of strengthening the nation. Most of the independent states opposed unification. Conflict became inevitable. The NUSA backed by nationalized Militech forces declared war on the Free States: Colorado, New Mexico, Wyoming, Montana, Arizona, Nevada, and Northern California. Washington, Oregon, and Idaho remained neutral in the conflict at the price of making some concessions to the federal government. Despite weapons and "security advisers" being given to the Free States by Arasaka, they were pressed hard by NUSA forces who were supported by states that supported federal intervention. This conflict used cutting-edge technology and was named "the Metal Wars."
Night City avoided the fighting but only barely. Northern California allied with the Free States while the South allied with the NUSA. Night City civilians held their breath as they waited for a federal invasion. In early 2070, an NUS Army division was prevented on the outskirts of Night City from invading it thanks to the actions of councilman Lucius Rhyne. Rhyne had used his decade worth of contacts on the city council and begged the long-shunned Arasaka for protection. Within days, an Arasaka supercarrier showed up in Coronado Bay. The NUS Army retreated hours later.
Following Arasaka's intervention, the NUSA and the Free States signed a treaty known as the Treaty of Unification, ending the Unification War. The treaty ensured the Free States would remain autonomous bu would have to participate in the new federal government and hostilities among themselves would have to cease. President Myers agreed to this treaty because she feared Arasaka's increasing involvement could escalate the conflict into one the New United States could not afford. The peace treaty was unsatisfactory to all parties but was considered preferable to continuing the war and risking another global crisis.
Night City was declared an international free city, free from the laws and governance of the NUSA and the Free State of Northern California. Night City paid for this liberty with an increase in megacorporation influence, the corpos pumped money into the revitalization of the city so they could have a foothold in the West Coast of the NUSA. This event took place in 2070, when Arasaka was allowed to build their new American headquarters downtown on the site where the previous one was destroyed in 2023. The new prosperity in Night City was not enjoyed by all.