Cyberwar is no longer a far-fetched idea. In fact, it has already happened.
Just a few weeks ago, hackers took control of at least 30 power stations in the Ukraine, leaving 230,000 people without electricity. It was nighttime, so it was dark, and it was the middle of winter, so they were freezing.
Hackers, likely Russians, had taken control of the power stations' computer systems. In fact, some employees saw their cursors moving on their computer screens, typing in new passwords, and shutting down their computers. The hackers left them helpless, and residents in danger.
It was the first confirmed hack to shut down a country's power grid. It won't be the last, according to Rogers.
He was speaking at the RSA cybersecurity conference. Cyberwar isn't his only concern, though. He's also worried that terrorist groups, such as ISIS, will attack the U.S.'s power system. The NSA chief also said he's worried about data manipulation, where hackers take over your computer and change the information on there.