Israelis take comfort in the belief that the IDF does all it can to spare civilians, but its shelling of Gaza calls that belief into question. Two kinds of artillery shells selected for use in Gaza - one as an "obscurant," the other as a weapon - were utterly inappropriate for the densely populated areas where the IDF deployed them. Their use violated the IDF's obligations under the laws of war. The first was white phosphorous. A typical shell releases 116 phosphorus-soaked wedges which, upon contact with oxygen, burn intensely, releasing a distinctive plume of smoke. That smoke can be used legitimately to obscure troop movements, but white phosphorous can be devastating when used in urban areas, igniting civilian structures and causing people horrific burns. Its use violated the legal requirement to take all feasible precautions during military operations to avoid harming civilians. The IDF never should have used it in densely populated sections of Gaza. The IDF has tried to defend itself with denial and obfuscation. It first denied using white phosphorous at all. Then, when that proved untenable, it claimed that use was limited to unpopulated areas. Neither claim is true. On January 9, 10 and 15, a Human Rights Watch military expert personally observed white phosphorous being fired from an artillery battery and airburst over Gaza City and the Jabalya refugee camp. Its telltale jellyfish-like plume was a dead giveaway, as can be seen from photographs. The Times of London also photographed an IDF artillery battery firing white phosphorous shells. The shells are color coded and labeled with the IDF term for white phosphorous - "exploding smoke." They are also marked with the code used by the US manufacturer of white phosphorous - M825A1. Similarly marked and color-coded shells and other evidence of white-phosphorous use have been recovered within urban areas of Gaza where they fell. As for obfuscation, the IDF claimed that all weapons it used were "legal," but that begs the critical question of how they were used. The use of white phosphorous is legal in certain circumstances but illegal when deployed in a way that causes unnecessary or indiscriminate harm to civilians, as the IDF's use in Gaza clearly did. The IDF even harped on press reports suggesting that the International Committee of the Red Cross supported its position, but in a rare public comment, the ICRC denied that claim. The IDF's latest line is that the shells fired in Gaza "contained phosphorus material, but were not actual phosphorus shells." That is semantic game-playing. Nothing that indiscriminately burns the way the IDF's shells did, regardless of name, should be used in densely populated areas. THE IDF'S USE of 155 mm. high-explosive artillery shells in Gaza was also clearly unlawful. These weapons can injure civilians from blast and fragmentation over an area with a radius of as much as 300 meters. In the densely populated residential areas of Gaza, where Human Rights Watch saw these shells used on January 15, they can cause extensive civilian casualties. Such use violates the laws-of-war prohibition of indiscriminate attacks because the shells strike military targets and civilians without distinction. Some might argue that the IDF's unlawful use of white phosphorous and high-explosive shells is justified by Hamas's deliberate and indiscriminate attacks on Israeli cities and towns. But illegality by one side to a conflict does not excuse illegality by the other. And as should be obvious, it is hardly in Israel's interest to degrade international law protecting civilians. The IDF holds Hamas wholly responsible for civilian casualties in Gaza, alleging that Hamas combatants stored weapons in mosques and fought from among civilians. Those allegations may or may not be true. Long experience, as during the 2006 war in Lebanon, shows that we must take such ritual IDF pronouncements with a grain of salt. We will not know exactly how Hamas waged the war until human rights monitors can conduct on-the-ground investigations. The IDF's refusal during the fighting to allow journalists and human rights monitors into Gaza suggests that it did not want its claims tested by independent inquiry. Israelis seem dismayed that the world has not embraced the justness of its latest war in Gaza. Of course Israel is entitled to defend itself from Hamas's rocket attacks, but when it does so in violation of its duty to spare civilians, and with so massive a civilian toll, public outrage is entirely predictable. The writer is executive director of Human Rights Watch.