The force marched nearly all night, and after a short rest, early next morning attacked a strong outpost of the rebels...On the banks of the Rio Grande. The enemy was driven in confusion from his position...The last volley of the war."

The correct answer to this open question is the following.

It seems there are no questions here. Just statements.

What is your question? What do you want to know?

Trying to help you we can comment on the following.

It seems that here you are referring to the last battle during the American Civil War. It was a small battle that practically nobody knows anything about. It was known as the Battle of Palmetto Ranch, fought on May 12-13, 1865 in South East Texas, close to Brownsville.

So according to the official Union's report, the last volley could have been fired as part of the last moments of the Palmetto Ranch Battle on May 13, 1863.