Like many ambitious young men, Soule traveled to Colorado Territory in hopes of making his fortune during the gold rush around Pike's Peak. When the Civil War broke out in 1861, however, he abandoned the search for gold to assist the Union. Lieutenant Soule's Colorado unit spent most of the war years working to prevent Confederate-aligned rebels from taking control of communities in the Southwest. He was later promoted to Captain and assigned to guard mountain passes along the Sante Fe Trail. In this capacity, Soule worked as a liaison with Native leaders and helped to frame several treaties between the Cheyenne and the Arapaho.
During a period of high tensions between soldiers and Native tribes at Sand Creek, Soule disobeyed an order to fire upon a Native village. His action saved many innocent lives as none of the soldiers under his command fired upon the villagers, who had either fallen to their knees or ran from the village. He would later be one of the first soldiers to testify against his superior officers. In retaliation, Soule was killed at this location less than three months after his testimony. The following selection comes from a letter by Soule to another army officer shortly after the Sand Creek Massacre.
I refused to fire and swore that none but a coward would. For by this time hundreds of women and children were coming towards us and getting on their knees for mercy. Anthony shouted, kill the sons of bitches Smith and Louderbeck came to our command, although I am confident there were 200 shots fired at them, for I heard an officers say that Old Smith and any one who sympathized with the Indians, ought to be killed and now was a good time to do it. The Battery then came up in our rear, and opened on them. I took my comp'y across the Creek, and by this time the whole of the 3rd and the Batteries were firing into them and you can form some idea of the slaughter. When the Indians found that there was no hope for them they went for the Creek, and buried themselves in the Sand and got under the banks and some of the Bucks got their bows and a few rifles and defended themselves as well as they could. By this time there was no organization among our troops, they were a perfect mob – every man on his own hook. My Co. was the only one that kept their formation, and we did not fire a shot.