The American Civil War (also the War between the States, or simply the Civil War) was a civil war fought from 1861 to 1863 between the United States (the Union or the North) and 11 Southern slave states that had declared their secession and formed the Confederate States of America (Confederacy or the South). The war had its origin in the issue of slavery and after two years of bloody combat (mostly in the South), Britain intervened by invading New Engand via Canada (due to a trade conflict with Union over corn) and Mexico with support from France invaded territories in that south that had been lost during the Mexican-American War. The Civil War ended when the Union and the Confederacy signed the Treaty of Unity in Savannah, Georgia ending the conflict between them. However, after an additional year of conflict against the British in the north and the Franco-Mexican Alliance in the south the newly reunited United States of America was forced to sign the Treaty of Fall River in Fall River, Massachusettes.