On November 29, 1947, the UN voted to partition Palestine into separate Arab and Jewish states. This was immediately followed by the Arab Higher Executive calling for a three-day strike and Arabs throughout the land began a series of deadly riots, with all Jews as targets.
The Arab League worked against the establishment of a Jewish state in any part of Palestine, and dark warnings were given about how the Arab street would end up targeting Jews in Arab lands if the state would be born. They started organizing volunteers from neighboring Arab countries to fight the Jews.
The Jewish community in Palestine found itself being attacked from literally all directions. Jews were slaughtered in Haifa, in Jerusalem, and in the Negev. Jews were also targeted in Arab countries, with synagogues burned in Aleppo and scores murdered in Yemen. Jews were attacked wantonly, and virtually the entire Palestinian Arab community wholeheartedly backed the violence against the Jews. Many Jewish communities were cut off from supplies as Arab bands ambushed Jewish convoys and suppy trucks on the roads.
The massacres continued into the new year, with dozens of unarmed Jewish orange growers murdered in January. Also in January, the Arab League started to send its new army into battle to capture Jewish settlements. By February, the British admitted that some 1400 Arab fighters had infiltrated from Syria and Jordan into Samaria, and were being hidden by the local Arab population.
There is no evidence of any Palestinian Arab calls for peace during this time period. While the Chief Rabbi of Israel appealed for calm, while the Haganah and the Jewish Agency emphasized that they wanted to live in peace with the Arabs, no reciprocal calls were made from the Arab side. It appears that the Palestinian Arab community was unified in its desire to do everything necessary to stop Jews from controlling any part of Palestine.
This is not to mean that the Arabs were unified in other areas. Arabs who ignored the strike in December were killed, and the Arab Legion could not agree on strategy. Power struggles broke out between the Arab League-backed Husayni militant faction and the Nejada faction, which ended up fleeing the country. Jordan's King Abdullah, long a friend of the Nashabishis, backed a more representative Palestinian Arab leadership while the rest of the Arab League-backed Amin Husayni.
The Haganah, for the most part, stayed on the defensive and only shot back at the attackers during the first few months after the partition vote. The IZL and the Stern gang were not so circumspect and they would attack both the British and the Arabs without regard to civilian casualties. They were roundly criticized by the mainstream Jewish Agency but their reprisals got bloodier.
Even as these events were happening, the first wave of Arabs started leaving Palestine. Upper- and middle-class Arabs started leaving Jewish-majority areas as early as the first week of December, 1947, and by the end of March over 100,000 Arabs had left their homes, either to other Arab towns that were slated to be in the Arab Palestinian nation or to other Arab countries altogether. For the most part, these moves were voluntary. Almost certainly, this first wave of Arabs fleeing Palestine included many of the same people who left during the 1936-39 riots and then returned - people who had the means and the opportunity to escape what was sure to turn into a war zone.
By the beginning of April, 1948, the Haganah had to re-evaluate its defense policy. Almost every major road between Jewish towns and settlements was dominated by Arab villages and virtually all Arab villages were hostile towards the Jews, strangling the Jewish supply routes. The war against local Arab terror and foreign Arab irregulars needed to be won before the anticipated May 15 withdrawal of all British forces and the expected invasion from the surrounding Arab armies. Politically, things looked bad as well, as the US State Department started making noises that it was not supporting the May partition. The Haganah needed to act to gain the upper hand as soon as the Zionist state was to come into being.
This meant that the Arab villages that were responsible for the convoy attacks, especially within the Jewish partition, would need to be neutralized as a threat. The status quo could not be allowed to go on.
The name of the new Haganah offensive plan was Tochnit Dalet, Plan D. This plan allowed for the military disarmament and, if necessary, destruction of any Arab villages that were considered strategically critical for a contiguous Jewish hold on its areas. While the plan itself could have been theoretically used for "ethnic cleansing" of the Arabs, in reality most of the village populations evacuated ahead of the Zionist offensive in April and May, 1948. There is no indication of a deliberate, planned Jewish policy of "transfer" of Arabs out of the Jewish state. (The entire concept of "transfer" was not considered as distasteful then as it is now; it was almost a given that there would be a transfer of Jews from the Arab state to the Jewish state after partition.)
On the contrary, in Haifa after the Haganah had already won the battle for the city in late April, the Jewish leadership assured the Arab citizens' safety - and the Arabs decided to flee anyway, even though the fighting was over. It appears that they were threatened, explicitly or implicitly, by the Arab Higher Executive not to stay under Jewish protection.
Another instructive episode happened in Tiberias. The Jews and Arabs of Tiberias enjoyed pretty good relations, but outside Arab fighters came to Tiberias and forced the evacuation of the local population in order to be able to fight the Jews there.
The Jews were not entirely blameless in the flight of the Arabs. There were some massacres, including Deir Yassin by the Irgun, but the number of Arabs that fled massacres directly were relatively small. Many, and probably most, of the Arab refugees fled because of wildly inflated rumors of Jewish massacres in neighboring areas. Later in the war the Arabs of Ramle and Lydda were encouraged to leave by the Jews after their towns were captured in July, but those areas had been war zones: the Arab Legion had declared an offensive based out of Lydda towards Tel Aviv after a four-week truce expired, with many Arab soldiers were surrounded and captured during the offensive. Significantly, when the mayor of Lydda offered to surrender, he was shot dead - by the Arabs.
The standard Zionist histories of the war usually say that Arabs were urged to flee by their leaders. While there is little contemporaneous evidence of this happening, reports about such urgings surfaced from a variety of sources, many of which were Arab or pro-Arab. It seems clear that this did in fact happen on a number of occasions, including times where women and children were told to abandon villages (temporarily) so that the men can fight more effectively. It would not exaplin the great majority of Arab flight, however.
In the end, there were many cumulative factors that caused the bulk of Palestinian Arabs to decide to flee: fear of rumored massacres, the loss of their most prominent citizens in the initial flight in 1947, the desire not to live under Jewish sovereignty, the mob mentality that (mostly illiterate) Arabs had where a trickle can quickly turn into a torrent, the sudden loss of jobs in Jewish enterprises, and inter-Arab fighting. Historians have listed these factors in explaining the mass exodus of some 600,000 people. Above all, the major factor that all agree on can be broadly categorized as "fear."
But there is another critical part of the analysis that has not been spoken about, to my knowledge.
In 1948, the Arabs who lived in Palestine still did not consider themselves "Palestinians." They were, above all, Arabs. The nationalism that was pushed by Husayni and their other self-proclaimed "leaders" never really took hold on them, and a large percentage of them were either born in neighboring Arab countries themselves or their parents were. The reason that many of them were in Palestine to begin with was not because of a deep attachment to the land or even because their ancestors lived there - they were there for purely practical reasons, for jobs and money and a good place to raise their families.
The hostilities that started in 1947 changed the calculus in their minds of where they should live. Itinerancy was in their blood, and the idea of moving to another Arab country was not so forbidding - the borders between Arab nations were a fairly arbitrary Western invention and the typical Arab did not recognize them. Arabs had moved freely within the Middle East for centuries and they would continue to do so for centuries more as necessary, and Arabs are famously generous to their guests.
In other words, Arabs were not only calculating the costs/benefits of leaving Palestine, but they were also calculating the costs and benefits of moving elsewhere, where they can start life anew yet again, in honor and dignity.
To understand better why the Arabs left Palestine, it is instructive to ask another question: why didn't the Jews leave Palestine? They were being massacred, they were facing war, they had an uncertain future at best They had no less fear than the Arabs who were abandoning their homes by the hundreds of thousands.
But there were two major differences between the Palestinian Jews and the Palestinian Arabs.
While the Arabs moved to Palestine for mostly practical, economic reasons, the Jews moved there for ideological and religious reasons. Simply put, the Jews had a deep love of the land, and the Palestinian Arabs at the time had very little.
Even more important, though, is the differences in the incentives to leave. The Arabs didn't think that there was much of a downside to leaving - they would either live with their neighbors for a short time while the Arabs destroy the Jewish state and then they would return, or at worst they would be able to start over in another Arab area. The Jews, on the other hand, simply had no other place to go.
It wasn't until the end of 1948 that the Arabs of Palestine started realizing that their calculus was terribly wrong. Instead of being welcomed by their Arab brethren, they were dumped into refugee camps; instead of this being a temporary situation where they would be able to move to their homes in Palestine, the Jews had no interest in welcoming back people who effectively supported their annihilation.
This was only the beginning of their problems. It is truly ironic that the beginning of real Palestinian Arab peoplehood came as a result of them losing their chance at nationhood.