What causes the united states to double in size in 1803

 THE LOUISIANA PURCHASE, when the United States purchased the Louisiana Territory from France.

Explanation:  

Initially, President Thomas Jefferson had commissioned James Monroe and Robert Livingston to negotiate a deal with France to acquire New Orleans or all or part of Florida, as a means of avoiding the potential of an armed conflict in such areas. Monroe and Livingston were authorized to spend up to $10 million.  What they found out was that Napoleon was already set to sell a much wider range of territory to the United States, to finance his European wars.  Napoleon was asking $22 million for the whole territory that became the Louisiana Purchase.  The US team negotiated the price down to $15 million.  The deal with France was made in 1803.

Then, however, there was a constitutional crisis back home.  Did the President have the authority under the constitution to make such a major addition to the nation's territory and spend the nation's funds to do so? Ultimately, Jefferson was convinced by his Cabinet members and sent the measure to Congress for approval.  In a statement he made at the time, Jefferson justified the purchase with this analogy:  "“It is the case of a guardian, investing the money of his ward in purchasing an important adjacent territory; and saying to him when of age, I did this for your good." 

. political or economical policy that isolates a country from participating in international relationships. it has been suggested that the united states participates in isolationism because of its lack of involvement in particular wars or alliances.

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