The United States is warning that a cyber attack -- presumably if it is devastating enough -- could result in real-world military retaliation.

Easier said than done.

In the wake of a significant new hacking attempt against Lockheed Martin Corp, experts say it could be extremely difficult to know fast enough with any certainty where an attack came from. Sophisticated hackers can mask their tracks and make it look like a cyber strike came from somewhere else.

There are also hard questions about the legality of such reprisals and the fact that other responses, like financial sanctions or cyber countermeasures, may be more appropriate than military action, analysts say.

"There are a lot of challenges to retaliating to a cyber attack," said Kristin Lord, author of a new report on U.S. cyber strategy at the Center for a New American Security, a Washington-based think tank.

"It is extremely difficult to establish attribution, to link a specific attack to a specific actor, like a foreign government."

The White House stated plainly in a report last month that Washington would respond to hostile acts in cyberspace "as we would to any other threat to our country" -- a position articulated in the past by U.S. officials. (read more)