The Republic of the Rio Grande. In 1840 several Mexican States attempted to follow Texas and break away from Mexico. Caohuila, Nuevo Leon, and Tamaulipas. For most of 1840, they attempted to follow in the footsteps of Texas, but failed.
The capital was located at Loredo Texas.
And the president was Jesus de Cardenas. They pleaded with the then Republic of Texas for aid, but none came. Texas just wasn't ready for a second round with Mexico. Texas would have to wait another seven years until 1846.
Some people think the flag may have been green instead of black? Nonetheless it was a distinct rebel nation to fly over Texas. It is older than the Confederate flag in any edition, and like the Confederacy a forlorn dream lost unto the ages. Gracefully an echo of this fallen nation has been conserved as the city flag of modern Loredo, Texas.