Would hacking by a foreign government be considered an attack?

"BEIJING/SAN FRANCISCO (Reuters) - A secretive Chinese military unit is believed to be behind a series of hacking attacks, a U.S. computer security company said, prompting a strong denial by China and accusations that it was in fact the victim of U.S. hacking.

The company, Mandiant, identified the People's Liberation Army's Shanghai-based Unit 61398 as the most likely driving force behind the hacking. Mandiant said it believed the unit had carried out "sustained" attacks on a wide range of industries.

"The nature of 'Unit 61398's' work is considered by China to be a state secret; however, we believe it engages in harmful 'Computer Network Operations'," Mandiant said in a report released in the United States on Monday."

And if it is an attacks, what can be done about it?