As written by Lycan When the Spartan II Program was revived over 10 million candidates were chosen. 5 million died in augmentation and only half of that number were fully compatable as soldiers. Now with nearly a million Spartan IIs the UNSC thought they were ready for anything... but the one thing they weren't ready for was a rebellion.
As the Spartan's numbers were increasing so was their cost. Government officials were increasing the budget to create Spartans everytime another candidate was selected. The UNSC argued with the Governament complaining that it was becoming too costly to create soldiers to help defend Earth. After an argument that lasted nearly 7 years up until a majority of Earth's MAC cannons went offline and a majority of Earth's continents were beginning to fall under covenant control, they agreed to make all the materials required to make Spartans free of cost. Thousands more Spartan IIs were made
Unfortunately some of the people responsible for reviving the Spartans and a majority fo the Spartans themselves felt offended. The otherwise minor delimma escalated into a full scale civil war, 2/3 of the Spartan graduates joined the URS and 3/4 of the Marines joined aswell