As arguments flare in Israel and the United States about a possible military strike to set back Iran's nuclear program, an accelerating covert campaign of assassinations, bombings, cyberattacks and defections appears intended to make that debate irrelevant. The campaign, which experts believe is being carried out mainly by Israel, apparently claimed its latest victim on [January 11] when a bomb killed a 32-year-old nuclear scientist in Tehran's morning rush hour. The scientist, Mostafa Ahmadi Roshan, was a department supervisor at the Natanz uranium enrichment plant. He was at least the fifth scientist with nuclear connections to be killed since 2007; a sixth scientist, Fereydoon Abbasi, survived a 2010 attack. Iranian officials immediately blamed both Israel and the United States for the latest death, which came less than two months after a suspicious explosion at an Iranian missile base that killed a top general and 16 other people. Neither Israeli nor American officials will discuss the covert campaign in any detail, leaving some uncertainty about the perpetrators and their purpose. Israel has used assassination as a tool of statecraft since its creation in 1948, historians say, killing dozens of Palestinian and other militants and a small number of foreign scientists. But there is no exact precedent for what appears to be the current campaign against Iran, involving Israel and the United States and a broad array of methods.