Dating to the American Civil War, the flag popularly known as "the confederate flag" was the battle standard used by the Confederate States of America's Army of Northern Virginia, and was also used as the CSA's naval ensign displayed as a smaller canton with a white field.
Contrary to popular belief, it was never used as the national flag of the Confederacy, nor was it used by any of the Confederacy's armies west of Virginia. It is sometimes mistakenly called "Stars and Bars", which was the nickname of the actual first national flag of the Confederate States.
After the war, the ANV battle flag was adopted by many Southern groups opposed to Reconstruction and a century later was used as a banner for groups opposed to Civil Rights and Integration, leading to its status as a divisive symbol today. In modern days, the flags are associated with white supremacy and racism in a similar vein to the Nazi flag.
All flags connected to the Confederate States other than the aforementioned flag could be also be given the 'confederate_flag' tag.
Examples
The "Stars and Bars"/National Flag of the Confederate States
