While the Revolutionary War established the precedent for the proper treatment of soldiers, the Civil War truly advanced first aid on the battlefield. This is due in large part to a man named Jonathan Letterman, who became known as the Father of Modern Battlefield Medicine.
After it took one week to remove wounded soldiers from the battlefield at the second Battle of Bull Run in the summer of 1862, General George McClellan gave Letterman, who was the assistant surgeon of the Army medical department, the freedom to do whatever it took to provide the men the care they deserved.
Letterman didn't waste any time. He created the country's first ambulance corps that consisted of a multi-stage process in which men would run onto the field during battle, retrieve the wounded and get them to a field-dressing station where his new system of triage -- in which men were tended to based on their likelihood to live or die -- was used. From there, men were moved to a field hospital -- usually a nearby home or barn -- if necessary and eventually to a large offsite hospital where they could receive long-term treatment without the chaos of battle raging around them.
The new, multi-step process where soldiers were given first aid directly on the battlefield was tested at the Battle of Antietam in September 1862. It was a resounding success as medical personnel were able to remove all of the wounded from the field within 24 hours. Letterman's system was successful at both the Battle of Fredericksburg and the Battle of Gettysburg, where thousands of wounded soldiers' lives were saved. His system was subsequently adopted for the U.S. Army by an Act of Congress in March of 1864.