The War of the Five Kings
Robb Stark is crowned King in the North, Renly and Stannis Baratheon both claim their brother's throne, Balon Greyjoy claims the sea, and Joffrey holds the chair itself. A red comet hangs over all of it. Every one of them has a decent legal argument and an army, which is precisely the problem.
The fuller tale
Within a year of Robert's death there were five men wearing crowns. Joffrey held the chair and the city. Stannis Baratheon had the law and almost nobody. Renly Baratheon had no legal claim at all and the largest army in Westeros. Balon Greyjoy crowned himself in the islands because the mainland was busy. And Robb Stark's own bannermen picked him up and shouted him King in the North, which he had not asked for and could not refuse.
Above all of it hung a red comet, which every faction in the story read as a sign meant specifically for them: the Lannisters saw Joffrey's, the northerners saw Robb's, the red priests saw their lord's, and a boy beyond the Wall saw the last of something else.
Every one of the five had a decent argument. That is what made it unstoppable. There was no usurper for everyone else to unite against, only five reasonable positions and five armies, in a realm that had never had a mechanism for choosing between them except dragons, and had no dragons.
The war on the ground
The fighting was mostly the riverlands, because the riverlands are always where the fighting is: no mountains, the best farmland in the realm, and the roads that link everything. Robb won every battle he fought ÔÇö the Whispering Wood, the Camps, Oxcross ÔÇö and it did not matter, because his capital was hostage, his father was dead, his brothers were reported dead, and his bannermen wanted to go home for the harvest.
The lesson of the whole war is that in Westeros battles are cheap and alliances are expensive, and Robb Stark was extremely good at the first and made exactly one mistake at the second.
Elsewhere in the wiki
 See the world as it stood in 299 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.