I'm sure its no accident that the Pentagon stated they may treat cyber attacks as "acts of war" - http://www.bbc.co.uk/news/world-us-canada-13614125
And they're clearly not treating it as an act of war, which is why the US (thankfully) isn't at war with China right now.
Politically, going to war with China just because they hacked the SecDef's email would be an impossible sell to the American people, who frankly aren't that keen on starting World War 3 unless it's absolutely necessary. If the Pentagon is getting hacked by China then the correct response is better security, not making threats, whether idle or serious.