Opinion_what is national security?

When Zhao yan, the Beijing-based researcher worked for New York Times, was arrested for claimly writing the story of the high official reshuffle, the government said it is due to the concern of national security.

Now when CNOOC, China's largetst offshore oil and gas producer, was bidding for Unocal, the eighth-biggest U.S. oil company, protestors including two Republican congressmen, asked the President to review the bid for the national-security concerns, too.

It reminded Amy the similar thing happened when Lenovo, China's largest computer maker, bought IBM's PC division. The reason is same, national-security.

Then my questions is what is national security?

Searching on Wikipedia, the largest worldwide online dictionary, the words refer to policy enacted by governments to ensure the survival and safety of the nation-state, including but not limited to the excercise of diplomatic, economic and military power in both peace and war.

I wonder if it is right for me to add words in the explanation: the excercise of political concerns.