Short-term consequences are that the majority of the Jewish population in the region either flees (war and possible massacres of Jewish civilian populations by the Syrian army or Palestinian militias) or is expelled during the war, and those that stay are very likely either heavily ‘encouraged’ to emigrate or outright expelled from the region after the war.
Palestinian refugees to Syria are very likely settled in the region.
IMO Syria does the same with region as Egypt did with Gaza and assumes ‘trusteeship’ over the territory ‘on behalf of the Palestinian people’ or some such and effectively governs the region while pretending that the All-Palestine Protectorate has any authority. It is possible that the All-Palestine Protectorate is kept around officially longer if its ‘official authority’ is split between territories controlled by Egypt and Syria and not de facto dissolved by Egypt in 1953 (doing so would allow Syria to organize another ‘Palestinian government’ in their territory and allow them to score propaganda points by doing so) and is perhaps even allowed to return to the territories it is supposed to govern (IOTL it was forbidden to return to Gaza by Egyptian authorities), but neither Egypt nor Syria are interested in giving it any real authority.
If the formation of the UAR still happens, then the All-Palestine Protectorate is dissolved as IOTL, though it is possible that the Palestinian territories of Egypt and Syria are formally incorporated into the UAR as Palestine and since the All-Palestine Protectorate claims authority over all of Mandatory Palestine the UAR would lay claim to these lands as well.
Either way, once the UAR dissolves (Nasser’s behavior of not sharing power or compromising with the Syrian elites makes it almost certain that this will happen IATL as well) the situation returns to what is was before 1958. Syria might organize another Palestinian puppet government in the territories it controlled, but nothing much would change.
Israel likely later takes the region in an alt-Six-Days-War; the war won’t occur at exactly the same time as IOTL with a POD in 1948 but the dynamics of the regions and the political actors involved make it likely that sooner or later another Arab-Israeli war will happen, which Israel will win.
Israel likely annexes the area and settlement will begin, with historical Jewish settlements like
https://en.wikipedia.org/wiki/Yesud_HaMa'ala likely reestablished first and the area around Lake Hula a priority target for settlements.
An interesting question would be the status of the region’s Arab population: would they made Israeli citizens or retain their Syrian citizenship?
All of this assumes that history proceeds more or less as IOTL, but I can think of three scenarios which could alter the region’s history in more profound ways:
1) as described above nothing much changes until 1963, but the
https://en.wikipedia.org/wiki/1963_Syrian_coup_d'état fails due to
https://en.wikipedia.org/wiki/Ziad_al-Hariri, who was the commander of the Syrian army units facing Israel and played a significant role in the Baathist coup of 163 succeeding by leading these forces to Damascus in support of the coup, not being in its IOTL position and another commander not supporting the coup (different war of 1948 would mean different military postings and military careers afterwards). Afterwards, the army is purged of many officers with pro-Baathist leanings and Syria either continues as a civilian republic or the next coup attempt brings the Syrian Nasserists to power and the UAR is reestablished in some form. Either way, the history of Syria and possible the region and Arab nationalism would be profoundly changed.
2) There are indications that the
https://en.wikipedia.org/wiki/Arab_Liberation_Army planned to massacre the 1.700 strong Jewish population of Safed if it could take the city and IOTL the Syrian army deliberately shelled the civilian parts of Sadeh’s Jewish quarter IOTL. If such a massacre happens, it would be by far the single biggest massacre in the 1948 of either side and incite public opinion in Israel against Syria, especially if coupled with other massacres of Jews in the region. It would also make public opinion in the West more sympathetic to Israel and increase support of Israel among the American Jewry as Israeli claim that they are defending themselves against an enemy looking to at least ethnically cleanse Mandatory Palestine of Jews in the 1948/1949 war gain a lot more credibility in their eyes. Syria becomes Public Enemy No 1 in Israel and Israel might be tempted to escalate to escalate one of the intermittent hostilities that occurred IOTL despite the ceasefire agreement into a full-scale war in order to gain more buffer territory against Syria, either before or after 1956. Also, with Israel focussed on Syria it might not participate in the 1956 war which would deprive Britain and France of their casus belli for intervention, or if Israel has gone to war with Syria before 1956 and won Nasser might not feel up for confrontation with Israel in 1956 and, while still nationalizing the Suez Canal, decide not to close the Straits of Tiran. Either way, the Suez War is likely butterflied away, which would change regional history in a myriad of ways and possibly make Syria and not Nasserist Egypt the leader of Arab Nationalism.
3) Emboldened by a more successful performance in the 1948/1949 war, Syria forms a defense pact with Egypt before the 1956 War and is thus drawn into the war. It is possible that this changes US opinion of Suez Crisis, and the war is regarded less as a British-French imperialist adventure that will damage US political objectives in the region but more as a pre-emptive strike against a dangerous Egyptian-Syrian bloc supported by the Soviet Union (the Soviets sold arms both Egypt and Syria in that period, but was much closer politically to Syria and Syrian communists had a strong presence in the Syrian army and government) and the US doesn’t pressure Britain and France to abandon the war.
