is ti possible to define the natonal interest of a state?

Answer 1

If you mean a state, such as a US State, no. They aren't nations.

If you mean a state = a country, then yes. However, national interests are generally broadly defined, such as control over ones own important natural resources (especially energy producing ones), protection of the domestic economy, protection of the citizens, etc.

An example was Russia's recent decision not to allow an American oil company (Exxon, I believe) buy a 51% stake in the largest natural gas company in the country. Allowing a significant percentage of the country's natural gas production and reserves to be controlled by a foreign company is obviously not in Russia's national interest.

The big problems come when national interests are mutually exclusive. For example, it's in the US national interest to have a positive diplomatic relationship with all other countries, because our firms do business everywhere. However, if the US must take an action to secure another national interest, that action may end up annoying another country. It's then the job of the US government to decide which interest must prevail, and to what extent.