Web sites in China are being used heavily to target computer networks in the Defense Department and other U.S. agencies, successfully breaching hundreds of unclassified networks, according to several U.S. officials.Classified systems have not been compromised, the officials added. U.S. authorities remain concerned because, as one official said, even seemingly innocuous information, when pulled together from various sources, can yield useful intelligence to an adversary."The scope of this thing is surprisingly big," said one of four government officials who spoke separately about the incidents, which stretch back as far as two or three years and have been code-named Titan Rain...