When the King Louis, Queen Eleanor and the French crusaders arrived in the Outremer, they arrived in Antioch. Here Prince Raymond wanted the French to aid him and his forces in an attack on the city of Aleppo. Louis was some jealousy issues with Eleanor and Raymond's relationship, and decided to leave the city and continue on to Jerusalem.
At Jerusalem the Crusaders decided to attack Damascus, the only reliable Muslim ally the Crusader States had. The attack failed and the Crusaders left, having delivered Damascus into the hands of Nur el-Din, a Muslim warlord who united Syria and set the stage for Saladin's rise.
So lets say that Raymond and Eleanor are more convincing. Louis decides to attack Aleppo along with Raymond and the remnants of Edessa's forces (the Fall of Edessa prompted the Crusade). Louis' decision to attack Aleppo forces the other Crusader states and the German forces already in Jerusalem to join the attack.
Nur el-Din leads an attempt to relieve the siege, but he is killed in the attempt, leaving the Muslim forces in Syria in disarray. With Nur el-Din's death the city of Aleppo falls to the Crusaders. It is throughly sacked and Thierry, the Count of Flanders, is made the Count of Aleppo, swearing alliegance to Baldwin, his half-brother and the King of Jerusalem.
With the victory over Aleppo there is another council, and competing proposals to liberate Edessa, or to attack Damascus. In the end there is no decision, and the crusaders disperse.
With the fall of Aleppo, the momentum in the Outremer starts to go in favor of the Crusaders. Damascus is basically cut off from any Muslim forces that could relieve it in the event of a Crusader attack. Furthermore, with the investment of a foreign count in Aleppo, that city is heavily fortified. In the 1150's a Crusader attack is launched on Damascus, and that city falls to the Crusaders. By 1160 the Crusader States control Syria, and are in a position to maintain themselves for a long time.