The Civil War The Civil War came about for many reasons. Among them, probably the biggest, was the Mexican war. This war was mainly about the U.S. trying to gain more spanish territory to complete its manifest destiny. President Polk had been eager to compromise with britian on the division of the Oregon Territory because the United States was already at war with Mexico. The war came about for several reasons. Maxican patriots were still angry with the american annexation over Texas. So we insisted that the Texas border would be the Rio Grande. But on the other hand the Mexicans insisted it would be the Nuces river. Finally, controll of the mexican government changed hands