The U.S. Army of the 19th century largely did away with the big hats, wigs, and ornamental elements of the military uniforms of the past century as time wore on, though they retained mostly blue uniforms of thick wool with shiny metal buttons. The army of the independent Republic of Texas adopted grey uniforms. The Confederacy also chose grey to distinguish its soldiers from the federal blues in the Civil War. It was still generally the kind of warfare where units lined up and marched forward into musket or cannon fire with deadly result, as seen in skirmishes such as Pickett's Charge.
None of the Civil War uniforms were very functional, especially in the climate of the American south and west. During the battle for Atlanta in the summer of 1864, hundreds of soldiers on both sides suffered from exhaustion and heat stroke in their wool uniforms under the Georgia sun. Soldiers suffered similar problems in the wars against Mexico and the Native Americans on the Great Plains and in southwestern deserts.
The turn of the 20th century brought with it the idea that uniforms should be made for utility and concealment. Previous U.S. Army uniforms hadn't taken the climate or terrain much into account, other than perhaps to mercifully adjust the wear or weight of their material for summer. In 1902 the U.S. Army, learning from Britain's experience in colonial Africa and India, adopted the khaki uniform, known as "drab."
In the following years, almost every western military ditched the traditional bright colors and adopted uniform colors that aided concealment in shades of khaki, brown, or grey. At the outbreak of WWI, only the French Army maintained colorful uniforms of blue coats and red trousers.
World War I -- with its grinding trench warfare and development of machine guns, tanks, and warplanes -- had a powerful role to play in this change. For the first time, military uniforms were not meant to be seen, and this shift became even more pronounced a generation later. When most Americans think of G.I. Joe serving in World War II, they picture the M-1943 olive drab uniform, which was designed with the cold, wet, and verdant climates of Europe in mind.
Uniforms made of tightly-woven cotton "Byrd cloth" were designed for service in the tropics to stop mosquito, flea, and leach bites and allowed for better cooling than wool. However, these uniforms took longer to make and field and there was an acute shortage of them. Some old soldiers found it hard to shed military formality completely in favor of utility. General George S. Patton famously insisted that officers under his command still wear their ties into combat.
The U.S. Army field uniform remained largely unchanged with only a few minor alterations until the 1980s. When we think of the Korean War, we often picture men freezing in olive drab fatigues and coats huddled around fires wearing their pile caps. Grunts wore generally the same green uniform made of cotton in the jungles of Vietnam. The famous "tigerstripe" camouflage uniform worn in the 1960s by American advisors and special operations units in Vietnam was never officially authorized, though it was effective in the dense jungle. It also mimicked the uniforms worn by South Vietnamese soldiers and allowed U.S. troops to blend in with their counterparts as well as the terrain.