Otherwise, such a code-based attack may not be able to get beyond the level of establishing beachheads. To launch a code-based invasion, supercomputers and good infrastructure are essential. Such an investment, while not as expensive as conventional warfare, still requires backing by a government. China, Russia, India, and perhaps Iran, all come to mind. Though they have the talent and resources, keep in focus that they would not readily share such technology with terrorists.
Terrorists become wild dogs with just slight changes in the ideological winds.
And, with governments as the main players, also we have to consider the vulnerability to electromagnetic attacks. Computer systems and networks are highly vulnerable to high-energy bursts of electromagnetic radiation. They can fry computers. The twin demons of malicious code and EMPs have entered the arsenals of foreign powers. But, the United States also has these capabilities.