In 2012, we learned the answer: Stuxnet, the malware that infected Iran's Natanz plant in a bid to slow the nation's nuclear effort, which was developed by the U.S. and Israel. Security researchers had speculated those governments were the most likely Stuxnet suspects, and a New York Times report in June confirmed it.
Flame, the name given network-sniffing, audio-recording, keystroke-logging malware that infected Iranian oil ministry computers, was discovered in May. At first, it wasn't entirely clear who was responsible, but by mid-June, a Washington Post report had confirmed it was another U.S.-Israel joint cyberwar effort. A few months later, Defense Secretary Leon Panetta announced that the age of cyberwar had begun, saying the U.S. military "has developed the capability to conduct effective operations to counter threats to our national interests in cyberspace."