This is hot topic in today's Arab/Israeli conflict. However, it is one of the biggest fabrications as well. Before the War of 1948, over 100,000 Arabs (mostly well-to-do Arabs, but followed by Arab peasantry) migrated to Beirut and Damascus on their own terms and could return to a "better off" situation (it had been reported in Arab newspapers that the War of 1948 would last 4-6 weeks). This mass exodus happened before either side raised a weapon.
Upon war, it became quite clear to Israel that this was a war of survival. The Haganah paramilitary group used scare tactics to force Arabs to leave Jewish areas, but many Jewish leaders publicly called for the Arabs to stay. Arab countries were not similar in this respect. Many Arab countries encouraged, often times using their own scare tactics, Arabs to leave. In the Arab's view, the War would be over quickly and the fleeing Arabs could return to their lives. In 1943, the Jordanian newspaper Al Urdun wrote,
For the flight and fall of the other villages it is our leaders who are responsible because of their dissemination of rumors exaggerating Jewish crimes and describing them as atrocities in order to inflame Arabs...By spreading rumors of Jewish atrocities, killing of women and children, etc., they instilled fear and terror in the hearts of Arabs in Palestine, until they fled leaving their homes and properties to the enemy."
Khaled al-Azm, the Syrian prime minister, said,
"We brought disaster upon a million Arab refugees by inviting them and bringing pressure on them to leave. We have accustomed them to begging...we have participated in lowering their morale and social level...Then we exploited them in executing crimes of murder, arson and throwing stones upon men, women and children...all this for service of political purposes."
Mahmoud Abbas, the head of the PLO, wrote,
"The Arab armies entered Palestine to protect the Palestinians from Zionist tyranny, but instead they abandoned them, forced them to emigrate and to leave their homeland, imposed upon them a political and ideological blockade and threw them into prisons similar to the ghettos in which the Jews used to live in Eastern Europe."
Another argument that often arises is the "massacre" at Deir Yassin. Unfortunately, 107 Arabs did die (most of them Iraqi soldiers), but the fabrication of this story by the Arabs is questionable. Iraqi soldiers arrived to this area in hopes of blocking the main road to Tel-Aviv. When the Irgun arrived (another paramilitary group) they found themselves ambushed by Iraqi soldiers who had dressed as women and were firing in and around innocent civilians. Another piece of evidence that shows the mentality of this so-called political nationalism.
By May 15, 1948 armies from Lebanon, Syria, Jordan, Iraq, and Egypt (along with volunteers from Saudi Arabia, Yemen, and Morocco, attacked Israel and outnumbered them 5 to 1. Israel was obviously on defense and pushed the Arabs back and claimed land that had been "promised" to the Arabs (which they had turned down on numerous occasions). There was nothing close to ethnic cleansing.
At the Rhodes armistice talks in 1949, Israel willingly offered all of the land back in exchange for a peace treaty. The Arabs, not surprisingly, said no and launched, over the next six years, over 9,000 terrorist attacks that would eventually lead to the war of 1956. At the Lausanne conference, Israel (again) offered the Arabs' land back. The Arabs rejected this again because it would be recognizing the State of Israel. Instead, the Arabs maintained the refugee problem and claimed that it would stay that way until the flag of Palestine flew over the State of Israel.
~Reference from David Meir-Levi, History Upside Down (pgs. 66-75)