Cyber-warfare has been on the national security radar for the last few years. In fact, the Defense Department’s Global Information Grid is one of the most frequently targeted networks on earth. However, more recently, there is a larger, growing concern over the safety and security of the basic electronic framework of the United States. This critical piece of infrastructure is known as industrial control systems, and is a basic layout of all that which keeps the country running smoothly. Things such as water-treatment facilities, dams, pipelines, and the US power grid are all operated using the technologies of the industrial control system. From a realist perspective, this threat is real, and largely imminent. The Internet is, in the eyes of the realist, an anarchic system, an international battlefield with no governing body, or police force. Thus, those wishing to do harm can do so both unfettered and unchecked - providing they are smart enough to do so.
I'd also add that the US is uniquely exposed to this threat for a lot of reasons. Our defense capabilities in the cyber world are significantly worse off than our conventional military capabilities, especially relative to powers like China and Russia. Moreover, well-executed cyber attacks are significantly harder to accurately trace, reducing any deterrence that would exist in conventional warfare. Aside from just state actors however, it's significantly more likely that non-state actors could cyberattack the US (and I think they probably already have) than that non-state actors could hurt us militarily. Lastly, given how technology-dependent our financial system is becoming, the uncertainty caused by even the credible threat of an attack could tank markets, let alone an actual attack on a bank or something.