The Great Wars are a series of epic conflicts in several versions of Masters of the Universe continuity. They frequently serve as a plot device, either to distinguish present-day Eternia from prehistoric times, or to explain the absence of reliable information about legendary artifacts such as Castle Grayskull.
The Great Wars are first mentioned in He-Man and the Power Sword. Writer Donald F. Glut put no thought into the history of the Great Wars; the premise was simply an easy way for him to present a concept without having to explain it. Accordingly, little is established about the conflict itself. It serves primarily to convey the notion that Eternia was once a highly advanced society until the wars reduced it to a barbaric, post-apocalyptic wasteland with little knowledge of the previous era. The characters are familiar with science and magic, but it is implicit that any technology or sorcery predating the Great Wars is inherently superior to modern works, and often incomprehensible to present-day Eternians.
When the Goddess presents He-Man with the weapons and equipment she has been guarding, she implies their quality by stating that they were created by scientists centuries before the Great Wars. Skeletor later claims that a "hole" between dimensions was opened during the Great Wars, through which he was thrown from his native dimension into Eternia. In King of Castle Grayskull, He-Man remarks that Castle Grayskull was built before the Great Wars.
The Masters of the Universe Bible describes a series of wars on the evil world Infinita, which only ended when the last of the two warring factions each activated their ultimate weapns simultaneously, sending several of the combatants' leaders through a space portal to Eternia, which they decided to conquer. After the Infinitan invasion is repelled, and to prepare for the day the evil might return the Council of Elders create Castle Grayskull to store their knowledge until a champion emerges to use it. These concepts were abandoned without ever being used in either animated series.
The attempted invasion of Eternia by the Evil Horde, as alluded to in "Origin of the Sorceress" and described in "Reunions," represented an existential threat to the planet. Given the strength of the Horde Empire and the effort that would be required to repel such a force, this would presumably be one of the most terrible wars in Eternia's history.
"Journey to Stone City" reveals that Vokan's people were great warriors who had themselves turned to stone when life became too peaceful, so that they could be awakened when their talents would be needed again. This implies that a time existed, centuries ago, when Vokan's people were needed, although the nature of that crisis is unknown.
The Masters of the Universe Classics franchise makes frequent references to the Great Wars in its attempt to forge a single history from elements of all previous continuities. In the biographies included with the action figures, the Great Wars are presented as a series of conflicts between the Evil Horde, the Snake Men, and the forces of King Grayskull, around five hundred years before the birth of Prince Adam.
The wars began as a rebellion by the ancient Eternians against the oppressive rule of King Hssss, who had conquered the planet three hundred years earlier on behalf of the Unnamed One. However, the arrival of He-Ro to Eternia attracted the attention of the Hordak, who arrived with his Hordesmen and soon challenged the Snake Men for control of the planet, escelating the violence. Eventually the three warring factions agreed to a temporary truce to construct the Three Towers. Eventually, with the aid of He-Man visiting from the future, King Grayskull and He-Ro turned the tide against their enemies at the first Ultimate Battleground. Hordak betrayed King Hssss, causing the Snake Men to be banished to the Void. Grayskull in turn successfully banished the Horde to Despondos, although he would ultimately fall to Hordak's magic.