Your challenge, with a POD after 1945, is for Pan-Arabism to become a successful political movement with a single united Arab nation controlling at least a majority of the Arab World by 2019.
Prevent the Arab front in the Cold War somehow, so that the West is not so committed to subvert any attempt at Pan-Arabism (seeing it, basically, as a cover for Soviet/Communist influence) whenever it emerges.
A lot could be done if the armies do not acquire the OTL political prominence in so many key Arab countries (critically Egypt, Syria and Iraq) with Arabist discourse used to legitimise the rule of authoritarian (somewhat) leftist military officers.
Unfortunately, I think 1945 is too late to reach a peaceful solution to the competing Arab and Zionist claims over Palestine that both sides can accept (this was probably possible, albeit increasingly hard, in the Interwar). The First Arab-Israeli War was highly traumatic for Arab societies and greatly contributed to shape Pan-Arabism toward its militaristic bent (Arab victory there, besides it low probability, would also do relatively little for the Pan-Arab cause since it legitimises existing Arab regimes, largely conservative ones).