The movie is set in 1865. The Confederates are using 1873 Springfield Trapdoor rifles, The Mexican bandit leader is using a 1873 Trapdoor Carbine, John Wayne is using a 1873 Colt Peacemaker and an 1892 Winchester rifle.
A number of times in the early part of the movie reference is made to the surrender of Lee to Grant as the end of the war. Although the surrender of Lee is now seen as the effective end of the war, at the time it was not, and would not have been considered such by most people (the Confederate major in the opening battle being an example). Organized military operations continued for a number of months after Lee's surrender.
After the Civil War (1861-65) when the cowboys are around the campfire, Webster talks of sending a letter and that it could go Pony Express. The Pony Express dissolved in October of 1861. It also did not go into South Texas where the cowboys apparently were traveling.
The characters, both Mexican and American, keep referring to the war against Maximilian as a "revolution." As the Juarez government had never fled Mexico during the intervention, and consistently insisted it was the lawful government, no loyal Mexican would consider the war a revolution; it was the expulsion of a foreign invader.
When John Henry Thomas and Colonel Langdon meet for the first time, Colonel Langdon makes reference to Colonel Thomas' cavalry being with Custer at Gettysburg. Thomas was also supposed to be at the Battle of Shiloh. Further, it is obvious from the beginning of the movie that Thomas was a colonel of volunteers (not a regular officer). None of the volunteer cavalry at Shiloh was at Gettysburg, so Thomas' unit could not have been present at both battles.
Looking closely, you can see the rifle of the tenth member of the firing squad. Also you can plainly see his shadow on the ground at a 10 o' clock position. There are several other shots that confirm the number.
At the beginning of the movie, Ann Langdon (Marian McCargo) is shown placing flowers on the grave of her husband. His date of death is shown as March, 1863. Later, Col. Langdon tells Thomas that his brother was killed by Yankee cavalry at Chickamauga. The Battle of Chickamauga was September 19-20, 1863.
When Colonel Langdon leaves to go ask John Henry to give up his horses to save the Colonels men, he leaves and the sun is still up. He is shown riding at a full gallop at sunset and arrives at John Henry's camp sometime around daybreak. John Henry and his men take the herd back at a walk and still arrive just before noon, covering the same amount of ground at a walk in less than 6 hours, that the Colonel traveled all night to cover.