Bernard Wasserstein, DPhil, DLitt, Professor of Modern European Jewish History at the University of Chicago, in his 2001 book Divided Jerusalem: The Struggle for the Holy City, wrote:
"On 9 April  a unit of the Irgun Zvai Leumi [The National
Military Organization] entered the village of Deir Yassin, a little to
the west of Jerusalem, and massacred at least 100 Arabs (reports at the
time said as many as 250), including many civilians, among them women
and children. The incident was widely publicized and caused a wave of
panic and demoralization to spread through Arab Palestine. Many of the
remaining Arabs in mixed areas of Jerusalem fled."
Baruch Kimmerling, PhD, Professor of Sociology at Hebrew University, and Joel S. Migdal, PhD, Professor of International Studies at University of Washington, in their 2003 edition of The Palestinian People, wrote:
"Dayr Yasin [sic]...would become the  war's symbol for the Palestinians. The village was one of several attacked by Jewish forces in April  in an attempt to clear the besieged roads leading to Jerusalem. That offensive was important in itself, since it marked the first time Jewish forces fought with the strategic goal of permanently ridding an area of Arab villages in order to insure the viability of their own settlements.
The sequence of events in Dayr Yasin [sic] is now scarcely disputed. The village's non-belligerency pact with local Jewish forces did not spare it being swept into the Jewish offensive to break the Arab stranglehold on Jerusalem. Following an intense battle between Palestinian militiamen and Irgun forces with some Haganah [the nascent Israeli army] mortar support, Palestinian forces departed and the Irgun entered the village on April 9 . In brutal acts of revenge for their losses, the Jewish fighters killed many of the remaining men, women, and children and raped and mutilated others. Those not killed immediately were ignominiously paraded through Jerusalem and then sent to the city's Arab sector.
For their own purposes, both Israeli and Arab sources later inflated the number of those killed to approximately 250. A recent study by a team of researchers at Bir Zeit University found that the figure probably did not exceed 120. But that does not diminish the depth of the atrocity or its short- and long-run effects. In the immediate aftermath, the massacre became the subject of intense public concern. Despite their active participation in the early stages of the battle for the village, which itself left numerous local families decimated, the Haganah commanders and central Jewish leadership distanced themselves from what had taken place and condemned it. The Arab media used Dayr Yasin [sic] as the focus of their claim that Zionism was innately wicked, and to rally Arabs behind the impending Arab invasion. Broadcasts and newspaper stories prompted popular mass demonstrations in Damascus, Baghdad, Cairo, and Tripoli, including attacks on the local Jewish communities."
Benny Morris, PhD, Professor of History at Ben-Gurion University, in his 1999 book Righteous Victims, wrote:
"The attack, on the morning of April 9, was carried out with the prior approval of, and in cooperation with, the Jerusalem command of the Haganah. Some 130 IZL [Irgun Zvai Leumi - The National Military Organization] and LHI [Lohamei 'Herut Yisrael - Freedom Fighters of Israel a.k.a. the Stern Gang] fighters took part. During the battle, Haganah machine-gunners stationed nearby supplied covering fire, and two Palmah squads [Strike force within the Haganah] in armored cars joined in the actual battle. Palmah squads also helped evacuate the wounded, and the Haganah helped the combatants with ammunition...
Deir Yassin is remembered not as a military operation, but rather for the atrocities committed by the IZL and LHI troops during and immediately after the drawn-out battle...
The Jewish Agency and the Haganah leadership immediately condemned the massacre. Deir Yassin became the one Jewish atrocity that it was permissible to write about -- and to condemn. The Haganah made great efforts to hide its part in the operation... And over the years, the incident came to be used in Arab propaganda to blacken the name of the Yishuv as a whole...
Deir Yassin had a profound political and demographic effect. Despite a formal Jewish Agency Executive letter of apology and explanation to King Abdullah, the incident seemed to push Jordan into the arms of those pressing for direct intervention by the Arab states, and to undermine the secret Yishuv-Abdullah agreement. It may also have contributed to the decision of leaders of other nations -- principally Egypt -- to join the fray. Certainly the news enraged Arab fighting men, and 'Deir Yassin!' became a rallying cry for combatants bent on revenge.
At the same time, however, the news of what had happened -- extensively covered and exaggerated in the Arab media for weeks -- had a profoundly demoralizing effect on the Palestinian Arabs and was a major factor in their massive flight during the following weeks and months. The IDF Intelligence Service called Deir Yassin 'a decisive accelerating factor' in the general Arab exodus."