The Red Army appears in the book Interesting Times. The original Red Army is part of a legend about the founding of the Agatean Empire. In the Legend, a man known as the Great Wizard caused the earth to rise and fight on the side of One Sun Mirror, helping him to unite the country and become the first Emperor. The army the Great Wizard made from the earth was called the Red Army.
Thousands of years later, a revolution was sparked by a book called "What I Did On My Holidays" written by the tourist Twoflower. The Revolutionary group, led by Two Fire Herb and Pretty Butterfly, called themselves the Red Army. As the Army needed a Great Wizard like the one in the legend, they sent for the Great Wizard who had guided Twoflower in his journeys. This wizard was Rincewind. The plan was for the Red Army, led by the Great Wizard Rincewind, to overthrow the Emperor.
However, unexpected interference in the Empire by the Silver Horde, led by Cohen the Barbarian, led to Rincewind's discovery of the original Red Army, a collection of Golems created by the first Great Wizard, and controlled by magical armour. Using this Red Army, Rincewind is able to help the Silver Horde to victory, and Cohen becomes the next Emperor.
The "Red Army" reference works in a lot of ways, and goes quite deeply:
The most obvious connection is that the term is now more or less strongly associated with various Communist revolutionary groups, notably the Red Army Faction of West Germany and the Chinese Red Army of the People's Republic of China. The former Soviet Union also adopted the name to formally refer to its own armed forces.
The "original" Red Army of golems refers to the famous Terracotta Army that consisted of thousands of incredibly well-crafted soldiers, horses, and chariots made of terracotta, a type of clay. They were part of the funerary arrangements for the Chinese Emperor Qin Shi Huang, who established the first dynasty of a unified China. His name now refers to the dynasty he established, which in turn has been appropriated to refer to the country and people in general (Qin is pronounced as "chin").
Now here is where it gets a lot less obvious (might in fact be stretching it) and a little more interesting. During the Yuan Dynasty, the period in which China was ruled by the Mongolians, various secret societies were formed with the aim of overthrowing the dynasty when it became too corrupt and incompetent. According to popular legend, around the same time, a rumour about a prophecy began to circulate, which more or less went along the lines of "when the one-eyed man of stone is unearthed, the dynasty will be finished." Now as it happened, one day, while performing drainage and maintenance at a canal, a one-eyed figurine was found buried in the silt. Whatever the cause, divine or mortal, prophecy or not, the foreign Yuan rulers were soon thrown out and a new dynasty, the Ming Dynasty, ruled by native Chinese, was established. One of the groups that participated in the revolt were referred to as the Red Turbans.