It is usually declared by a group of nations against another one, in order to isolate it and to put its government into a difficult internal situation, given that the effects of the embargo are often able to make its economy suffer from the initiative.
The embargo is usually used as a political punishment for some previous disagreed policies or acts, but its economical nature frequently leaves space enough for doubts about the real interests that the prohibition gives advantage to.
Although the law of the United States does not prohibit participation in an embargo, it does prohibit participation in a secondary embargo. This occurs when one country pressures a business to stop doing business with a third country over issues which the business is not directly involved. Not only is an American business required not to participate in a secondary embargo but is also required to report all attempts to get a business to participate in a secondary embargo. The situation which led to these laws are attempts by Arab countries to prevent American companies from doing business with Israel.
The typical reaction is the development of an economical autarky.