The Clan Wars is the name of a timeline of events that has been ongoing since the Late Merethic Era, when human settlers first came to Tamriel. During times of struggle and survival some families with great expectations for wealth, power and honour, banded together and founded clans. This event has since been known as the "Founding of the Clans". Yet some clans became easily disgruntled when they saw how wealthy or powerful some of the other clans were and so decided to wage war upon them, sowing distrust and bigotry amongst them.
Some clans has remained allies and steady friends throughout the centures, others have forgotten about ancient wrongs and become neutral towards each other or chosen to co-operate, while others have chosen to continue their hatred for one another until the end of time.
The First Clan War
The First Clan War took place at the end of the Late Merethic Era and was sparked by reasons forgotten. One feud among two clans eventually grew into a nation-wide conflict with almost every clan founded back since the Late Merethic Era involved. Family's were massacred in their sleep and entire armies clashed on the plains and tundras of Skyrim.
The battle would continue on for decades and the clans which met their ultimate fate that day have not been recorded. The struggle eventually faded with the founding of the Alessian Empire and the defeat of the Ayleids.
The Second Clan War
The Second Clan War began during the decline of the First Era 1E 2840 in response to the Four Score War between the Reman Empire and Morrowind. Clans supportive of the Dunmer cause and mainly consisting of Mer attacked in retaliation and without warning the clans of Men. Though the clans of Men were greater in number and strength, the attack took them by surprise and wiped most of the minor clans out. Things seemed to be swaying in favour of the clans of Mer. It was by mere chance that Clan Blood Raven managed to rally their scattered armies and push the invaders back with the assistance of their cadet branch Clan Birds-of-Prey. Even though victory was achieved, many clans suffered great losses as well as many other clans being completely extinct, including Clan Birds-of-Prey.
The last battles wouldn't begin to subside until the late beginning of the Second Era.
The Third Clan War
Also known as "The Great Refusal" was a bloody conflict that lasted for generations. It began during the beginning of the Third Era with the death of Tiber Septim in 3E 38. The Emperor's death caused a lot of confusion between the clans loyal to the Empire's cause and those who only saw profit with the death of the man who had united Tamriel against their will.
Even if the bloodiest conflicts were fought and resolved early during the Third Era, the complete war wouldn't end until centuries later. The death of Jagar Tharn and the return of Uriel VII in 3E 399 marked the end of a three centuries long struggle.
The Fourth Clan War
The Fourth Clan War or "The Blood Feud" took place during the end of the Third Era, shortly after the Oblivion Crisis. With the Empire in crumbles, many clans who wanted to break free from the Imperial law and reign saw fit to do so, much to the dismay of other clans that wanted to hold Tamriel's clans together as one. Clan Bone Obelisk ultimately saw fit to be the first to break the rules and attacked their ancient enemies Clan Jackal, which started a chain reaction in which every clan was pitted against each other.
The feud eventually ended with the destruction of Vivec City and the eruption of the Red Mountain in 4E 05, in which many clans originating from eastern Tamriel suffered major losses and were rendered unable to fight further.
The Third Aldmeri Dominion
With the Great War in 4E 171 many clans deemed that an Aldmeri victory was to be avoided or else there would have been massive changes in leadership, loyalties and status among the clans themselves. Clans supportive of the Aldmeri cause however, could only see profit, and thus began an inoffical "fifth" clan war which is still ongoing up to date with more or less belligerents involved.