Someone above asked whether there is any country where there is no protection against unlawful dismissal, and the answer is probably not. But in the U.S. we have "employment at will" which means that you can be fired at any time, or quit at any time. What can make it illegal is if you are fired for certain specific reasons including race, gender, or age.

When you get "layed off" (which is a mis-use of the term, because in a union shop if you get layed off then when they start re-hiring it's done by seniority) from a big company they give you information about the demographic mix of the reduction, to show that the reduction of each group was taken in the correct proportion.

Regarding large American companies and how easy it is to get rid of people, perhaps a good current example is Hewlett Packard. HP is undergoing some problems and their new CEO recently (July 20) announced that the company would be cutting around 14,000 jobs around the world. Some people have already been notified, three weeks later, which means that they'll be out by the end of August.

There is NO requirement for prior annoucement except in certain cases where the local goverment is notified about plant closings.