Jon Arryn III

Jon Arryn III was Lord of the Vale during the second Clash of Kings. Shortly after the death of King Jon Arryn II, he got married to the (much older) Lady Tully, therefore forming a strong alliance with the Riverlands. Scholars remain confused about this marriage up to this date, because it seems that the Arryns provided all the power and wealth in the alliance and the Tullys did not actually bring anything to the table. Perhaps they were just very good friends and that was enough. 
Arryn and Tully also entered into an alliance with Lord Stannis Baratheon, especially because of shared concerns about rumours of a conspiracy between The North and The Reach, but this was not always the strongest alliance, because Lord Baratheon was too power hungry and was ready to betray the alliance if it meant becoming king. Later on, however, Lord Tyrell of the Reach switched sides to Arryn's alliance, whereas Lord Stark of the North formed a powerful counteralliance with the Westerlands, Dorne, and the Iron Islands. 
When Riverlands spies found out that Dorne had tried to stage a red flag against The North in the Riverlands, Arryn and Tully saw this as a war crime. Nevertheless, they preferred peace over war and invited Lady Martell of Dorne to King's Landing to talk it out peacefully. Lady Martell, however, brought along Lord Stark and refused to talk it out privately, and Arryn and Tully realised that peace was never an option and declared war on the The North-Westerlands-Dorne-Iron Islands alliance. 
At first they seemed to be losing the war, mainly because of a lack of power and defense in the Riverlands. Arryn was able to conquer a lot of land in The North, including Winterfell, but as soon as Lord Stark died, Lord Greyjoy sent his army to The North and scared Arryn away. However, Lord Baratheon ultimately proved to be a valuable ally after all, as he won over the Iron Bank and was able to get a lot of mercenaries to balance out the power difference. Because of these mercenaries, the alliance was able to destroy both the remaining armies of the North and Westerlands, and the counteralliance gave up and choose Lord Baratheon as King of the Seven Kingdoms along side Lady Tully and Lord Arryn. Lord Arryn's daughter would go on to marry the new Prince, Lyonel Baratheon giving House Arryn a place in the royal family.