continued....
Eventually the French sold the Louisiana area to the Americans in something called the Louisiana Purchase.
http://en.wikipedia.org/wiki/Louisiana_Purchas e
As has been stated, because these lands were almost totally uninhabited (except by native americans) and the land mass was huge, it was easy to create new states, you mostly used a ruler !
When this land in the West became available for use by the people in the East there was a huge move of people to the West, and because it was rather wild and untamed, for a while it was known as the Wild West, and spawned hundreds of cowboy films.
In fact we tend to think of the USA as one country, but if you think of the NAME of the country it is the United States of America.
In fact before the USA was formed by those original 13 or so states in the east each state had their own language, their own money system and so on.
It took a lot of effort to unify them as one country, rather like is happening in Europe now, with many different countries but an attempt to have a standard currency, and common laws and so on, a United States of Europe.