The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Patrol_cap
U.S. Army Rangers wearing patrol caps, 1986
A patrol cap, also known as a field cap, is a soft kepi with a stiff, rounded visor, and flat top, somewhat similar to a baseball cap, worn by the military personnel of some countries in the field when a combat helmet is not required.
M1951 Field Cap and Ridgeway Cap
Two soldiers wearing the M1951 Field Cap-Anyang South Korea, Lt. Green and Captain Ray
The M1951 Field Cap, introduced with the M1951 Uniform, was a derivative of the M1943 Field Cap, part of the M1943 Uniform. The M1951 cap was worn in the Korean War, where it became known as the "patrol cap" by the US Army Rangers there. It was constructed of wind-resistant olive drab cotton poplin, and had a flannel wool panel that folded down to cover the ears and the back of the head. It was soft enough to be worn underneath an M-1 helmet. After the Korean War, the cap was replaced by the Ridgeway Cap, a stiffened version of the M1951. The hat became famous outside America by being worn by Fidel Castro. The patrol cap was replaced altogether in 1962 with a baseball-like "Cap, Field (Hot Weather)" and during the Vietnam War by the boonie hat.
On June 14, 2011, the M1951/ACU soft patrol cap became once again the primary headgear for all Soldiers as the duty uniform headgear after a 10-year hiatus in favour of the beret, according to Army Directive 2011-11.
BDU, ACU and ABU patrol cap
Two soldiers wearing the ACU, as well as ACU-patterned patrol cap (left) and boonie hat (right).
USAF Captain wearing Airman Battle Uniform with digital tigerstripe-patterned patrol cap
To read more about patrol caps, please click on the following link: http://en.wikipedia.org/wiki/Patrol_cap