This is one of those issues that is still in dispute. The actual truth may never be known, however, if one looks at both sides claims then the truth is probably somwhere in the middle.
In early April, 1948, Jewish and Palestinian forces clashed in the town of Deir Yassin, in Palestine. It is generally accepted by most historians that Jewish forces led by Menachem Begin's Irgun massacred a large number of Arab civilians there. However, some supporters of Begin have rejected this claim.
Many accounts also state that the purpose of this massacre was strictly an exercise in propaganda. By parading the victims who survived in their bloddy nightgowns, through the neighbouring towns, they were able to further frighten and panic the remaining inhabitants of these areas. Also the sounds of the attack were recorded, and used again and again to frighten other villages, and areas, containing the native Palestinian population, this also encouraged them to flee.
A columnist who disputes the claims of a massacre, Sid Zion, asserted in a March 23, 1998 column in the New York Daily News that what happened in Deir Yassin was a pitched battle, not a massacre, and that the Irgun actually warned the Arab residents prior to the battle. He further concludes that the charge of "massacre" was a "libel". To support his assertion that the claims of a massacre were exaggerated, see the web site http://www.zoa.org/pressrel/19980325a.htm .
However, other Zionists disagree with this, and argue that to deny that a massacre took place is to rewrite history and to participate in something analogous to holocaust denial. For an example of a Zionist web site that memoralizes the victims at Deir Yassin, see the following: http://www.ariga.com/peacewatch/dy/dypail.htm . This web site strongly disagrees with the ZOA assertion, arguing that "Examination of the known history, including testimony by Irgun and Lehi combatants, indicates that this claim is absurd." The web site also includes an eyewitness account of the events at Deir Yassin: http://www.ariga.com/peacewatch/dy/dypail.htm