Also, just to piss people off just that bit more, I found this on Wikipedia:
Direct attack
Several modern-day armies operate nuclear weapons with ranges in the thousands of kilometers. Mainland U.S. is therefore vulnerable to nuclear attack by powers such as the United Kingdom, Russia, China, France, Israel, India, and Pakistan.Cyber and economic attacks
The risk of cyber-attacks on both civilian and military computer targets was brought to light after China became suspected of using government funded hackers to disrupt American banking systems, defense industries, telecommunication systems, power grids, utility controls, air traffic and train traffic control systems, as well as certain military systems such as C4ISR, and ballistic missile launch systems.
Attacks on the US economy, such as efforts to devalue the dollar or corner trade markets to isolate the United States is currently considered another method by which a foreign power may seek to attack the United States.