On 16 March 1968, Hugh Thompson, Jr., Glenn Andreotta, and Lawrence Colburne did their best to protect Vietnamese civilians from being slaughtered. In return for their heroism, they were accused of being traitors to the United States of America.
Helicopter pilots, they spotted the My Lai massacres taking place. Seeing a bunch of cowering women and children, and US troops fast approaching to slaughter them all............they landed their chopper directly between the troops and the Vietnamese Group........with the implicit threat that they would open fire, if the troops did not back off.
They evacuated the Group, and later returned to check the ditches full of dead bodies, to rescue any survivors.
That is but ONE famous incident, where US troops were prepared to fire on other US troops.
In the course of wars......there have been countless "friendly fire" incidents.........firing on your own side, or that of your ALLIES........fighting WITH YOU.