The Timeline

=== The Ascension of Bran the Broken ===
In the first Great Council meeting of Westeros, Bran the Broken, also known as the Three-Eyed Raven, was elected king of the 6 kingdoms with the North becoming an independent kingdom. Being the Three-Eyed Raven, Bran the Broken ruled for an unnatural long time on his throne. Scholars agree that his reign lasted more than a century and after 120 years of his rule he passed away peacefully in his sleep. Or he fell out of his wheelchair, sources vary.
During his reign more and more of the kingdoms demanded their independence and so to prevent war, King Bran gave this independence freely. This however meant that once independent, the kingdoms could no longer partake in any decisions the Great Council made. While nearing the end of King Bran's rule many nations came back into the fold including the North. It seemed that the struggle of kingship was more important than independence.
===The First Clash of Kings ===
When the time came to choose a new king, many nations staked their claim to the throne. Without a king like Bran there was also room for more conflict than ever before. Harrenhal came once again to the forefront of conflict when the new power in the Crownlands, the Crusader Queen, demanded the fortress for herself in order to spread the Light of the Seven there. During Bran's reign however his loose policy's had allowed for a new ruler of the Riverlands to rise up against the noble house Tully and was replaced by a Union of all territories in the Riverlands. The Trident Union, led by Joseph Dondarrion, had both military and religious reasons to not let Harrenhal fall into the hands of the Crusader Queen. After multiple incidents that were mostly caused by the Trident Union the Great Council, at the time consisting of Lord Greyjoy of the Iron Islands, Lord Arryn of the Vale and the Crusader Queen of the Crownlands declared war upon the Trident Union. 
The following conflict matched up the Trident Union and the Westerlands against most of Westeros. So after a bloody war, the Trident Union surrendered and gave up Harrenhal to the Crownlands. This conflict had sparked many old feuds and while the entire country prepared for a much bigger war, it all came to a sudden halt. Jon Arryn II was declared king by his fellow great council members in a hastenly crafted tactic of diplomacy, making him the First King after Bran the Broken. Without much military support on his own part to support this claim, the Trident Union and Crownlands still declared war and severely weakened both kingdoms, destroying one another in the process. 
House Tully grabbed it's seat in Riverrun once more and the Crownlands were now ruled over by King Jon Arryn II. His reign didn't last very long however and was found dead in a spikepit of the Red Keep only two and a half years into his reign. 
===The Second Clash or Clash of Councils ===
After the death of Jon Arryn II, the Great Council came together once more to decide on there new king. Lord Baratheon, Tyrell and Tully summoned every great lord in Westeros to King's Landing to help them pick a candidate.
After a week of not being able to decide on a king almost all the kingdoms who were not in the great council aside from the Vale started plotting a rebellion against the Council. This rebellion officially started after the council tried to intervene in border incidents started by the Lannisters against the Reach. Lord Haatack Stark made threats of using dragons to conquer Westeros and raised a massive army along side his allies. Being greatly outnumbered, the Council quickly lost it's foothold in the Reach, rendering it useless and out of the conflict after only the first few battles of the war. 
The tide soon turned however when House Bolton joined the Great Council at request of Lord Stannis and the Northern Army was cut off from it's homeland. Shortly after this show of loyalty by House Bolton Lord Haatack died from a mysterious plague that had taken root amongst his ranks. The rebels still had a sizeable force in the field and still had a chance to win the war. Lord Stannis Baratheon II eventually defeated the rebels with a united force of soldiers from the Vale, Riverlands, mercernaries and his own army from the Stormlands. After the final battle at Riverrun he was crowned king of all Westeros by both his defeated enemies and his allies, making him King Stannis Baratheon II, the Second King after Bran the Broken.
===The Third Clash or Clash of Chaos ===
The 3rd instance of a major power struggle within the Broken Kingdoms marked a complete change for Westeros with effects that had both direct and long lasting consequences for the continent.