The Pentagon has quietly empowered the United States Cyber Command to take a far more aggressive approach to defending the nation against cyberattacks, a shift in strategy that could increase the risk of conflict with the foreign states that sponsor malicious hacking groups. Until now, the Cyber Command has assumed a largely defensive posture, trying to counter attackers as they enter American networks. In the relatively few instances when it has gone on the offensive, particularly in trying to disrupt the online activities of the Islamic State and its recruiters in the past several years, the results have been mixed at best.