According to the International Law, national territory refers to a given part of the surface of the earth under the jurisdiction of a country as well as its underground and overhead air space. National territory is composed of territorial land, waters, air and the substratum. The territorial air refers to the air space above the territorial land and waters below a certain altitude under the jurisdiction of national sovereignty. The "Paris Convention" of 1919 recognizes that each country has complete and exclusive sovereignty over the air space above its territorial land. Therefore, as far as our defense is concerned, it includes not only land defense and coastal defense, but also air defense.

　　History shows that air defense came into being gradually along with the large-scale use of aircraft in military struggle. Along with the development of aviation forces, especially the establishment of the air force as an independent service by various countries in the world, and along with the development of aerial defense weapons such as aircraft, anti-aircraft artilleries and radars, etc, and the construction of various aerial defense facilities such as airports and air defense positions, the national defense over the air space has become a comparatively complete defense system. Today, air threat has become the primary important military threat towards national security, which can affects the overall strategic interests of a country. From a number of information local wars occurred after the Gulf War, we come to see that the wars tend to be increasingly fought in the air. The air powers with aerial-based information support have displayed tremendous operational advantages and played dominant roles in relatively independent air battles such as the Kosovo War. Therefore, for the countries in the world, especially those that pursue a defensive strategy, only by giving air defense building a top priority in their national defense, can they build up a strong air defense force to ensure their entire national defense security. This will play an important or even decisive role in seizing and maintaining the national strategic initiative, safeguarding the safety of key strategic targets and resources, ensuring the smooth transition of the country from peacetime footing into wartime footing as well as maintaining the initiative in the whole process of war and eventually winning the war.

　　By Ji Yan and Min Zengfu

　　(Feb.13, PLA Daily)