Based on what we've seen, an attack on by an enemy country's on a government or military network isn't viewed as gravely as an enemy launching mortar shells at a command post. The latter would almost certainly lead to a shooting war, whereas the former has been tacitly accepted as espionage rather than the opening of outright hostilities.
But what happens when enemy (or unfriendly) countries launch cyber-attacks that damage physical infrastructure (as the Stuxnet virus is widely believed to have done in Iran), or terrorists cause economic damage? Is it time to fuel up the bombers?
This is an issue that many countries are struggling with now, as this ComputerWorld article notes.There aren't yet any known rules of engagement for the U.S. military, for examle, for dealing with cyber-attacks (at least as far as we know) and it's unclear how a major power like the United States would deal with a massive hacker "Pearl Harbor," said panelists at the Techonomy 12 conference, according to the article.
The only historical example that even approaches something of this magnitude was the alleged CIA
involement in the explosion of a natural gas pipeline in Siberia
in the early 1980s. The U.S. intelligence agency reportedly used a "logic bomb" to interfere with a pressure test on the pipeline, leading to a massive (3-kiloton) explosion. However, both U.S. and Russian authorities have rejected these allegations.