Historically, "friendly fire" - the inadvertent shooting by troops on soldiers from their own side - has always been a feature of military operations. Certainly, the phenomenon is not a novelty for the IDF. The very first of Israel's generals to be killed in action, US Army Col. Mickey Marcus, was shot dead in June 1948 by one of his own men, who failed to recognize him. Since then, of course, communications technology has vastly improved, and so has sensitivity to the need for precautions and - above all - for adherence to fire discipline. Nevertheless, no army in the world has found it possible to entirely eradicate friendly fire or to avoid its consequences in terms of people killed and wounded - especially not when fighting in urban areas, where soldiers have extremely low ranges of visibility. The IDF experience in Operation Cast Lead is hence by no means exceptional. Of course, the statistical inevitability of some casualties by friendly fire during military operations gives no comfort to the victims' families and friends. They will forever believe that of all the possible fates awaiting their loved ones, this was by far the most wasteful and avoidable. If experience in Israel is anything to go by, the mothers and fathers of the dead will be especially vociferous. Long gone are the days when bereaved parents were prepared to take the IDF's version of events at face value. Some will insist on an independent inquiry; others will demand that any individuals found to have been negligent be appropriately punished. How Israeli society as a whole reacts will depend on a number of variables. The first, and most obvious, is the quantity of friendly fire incidents. A handful can be tolerated; more than that and people will begin to suspect that there is something fundamentally wrong with IDF training and discipline. Then there's the number of casualties. One of the characteristics of modern warfare is that, even when the overall number of friendly fire incidents is low, the damage caused by each individual incident can - thanks to the destructive power of modern technology - be extremely high. Its societal impact is accordingly magnified. Thus, a single helicopter collision over the Galilee in February 1997 caused the deaths of 73 IDF soldiers. The "Helicopter Disaster" in turn triggered the Four Mothers movement and a groundswell of support for their demand that Israel withdraw from southern Lebanon. In the last analysis, however, less tangible factors are likely to have the greatest impact. That, certainly, is the lesson taught by British, French and American public reactions to friendly fire incidents in numerous conflicts since 1945. In each case, the question that most people asked was not, "Can we bear these losses?" but "Why were the soldiers sent to fight in the first place?" The lesson is clear. Even in democracies, public opinion will tolerate relatively high levels of troop casualties - provided people are convinced that the fighting has a purpose, in the sense of being carried out with a clear political end in view. Once there exists suspicion that the government has resorted to force simply because it cannot think of anything else, even a small number of losses could become unacceptable. The writer teaches political studies at Bar-Ilan University and is senior research associate at the BESA Center for Strategic Studies.