A bit of backstory first. When the crusaders showed up in the Near East, the entire region was divided and at each other's throats.

In theory, there were two major powers: the Shi'i Fatimids in Egypt and the Sunni Seljuqs with their capital in Baghdad. However various governors all over the place were effectively autonomous and guarded that independence jealously. For example, after the First Crusade, the Seljuq sultan in Baghdad ordered expeditions dispatched to kick out these intruders. This was dispatched from their base in Mosul - however cities of Aleppo and Damascus were key logistical hubs and were needed in order to be able to attack the crusader states. Depending on how secure those cities felt, they might co-operate with the Mosul forces or they might ally with the crusaders to protect their independence. On top of that there were two Shi'i schisms so in addition to the Isma'ili Fatimids, you had the Druze and more famously the Nizaris ('assassins') who wanted to topple the Fatimids. So the Muslim world was completely fractured and very vulnerable.

The governor of Mosul, Nur al-Din figured out that he needed to secure both the key cities of Aleppo and Damascus in order to be able to prosecute a war with the crusaders so he began by capturing both. He also began to patronize religious scholars and jurists and they resurrected the ideology of jihad which had fallen into abeyance (there had been some usage of it in the 9th C against the Byzantines). With a secure north and a growing religious surge behind him, Nur al-Din then got involved with toppling the Fatimids who were in the middle of a civil war and had called on both him and the crusaders. Long story short, he won and eventually had the (to him heretical) Fatimid caliphate dissolved. Then he died.

This left the Kurdish Salah al-Din (formerly Nur al-Din's vizier in Egypt) an opportunity. He quickly moved north and pushed the heirs of Nur al-Din back to Mosul and secured Aleppo and Damascus. So now he commanded an immense amount of resources and had surrounded the crusaders. Up to this point he had a truce with the crusading states (and he used this security to secure his control over Syria). A raid on a pilgrim caravan by some douchebag crusader lord gave him both the casus belli and he began besieging Tiberias. This drew out the crusader army and he immediately harassed their march with horse-archers and stopped them short from a water source. The next day, thirsty and demoralized, he defeated the crusaders utterly.

The crusaders had (unwisely) stripped their garrisons bare to put together this army and now they had little to no force to defend their possessions. So Salah al-Din then just romped through the various states taking the cities back. Jerusalem fell after a (brief) 1-2 wk siege. Only Antioch and Tyre were left in Latin hands.

So to put a long story short - yes. His command of Syria and Egypt gave him access to wealth and manpower and more importantly denied the crusaders allies.