http://mobile.nytimes.com/2016/12/09/us/obama-russia-election-hack.html
“When warranted, the United States will respond to hostile acts in cyberspace as we would to any other threat to our country. We reserve the right to use all necessary means — diplomatic, informational, military, and economic — as appropriate and consistent with applicable international law, in order to defend our Nation, our allies, our partners, and our interests.”
And see:
https://global.oup.com/academic/product/cyber-war-9780198717508?cc=us&lang=en&