From late May 1786 to September 1788, Jay of New York, Rufus King of Massachusetts and other members of the Northern Congress argued that the critical state of American trade and foreign relations justified a 25-year navigation of the Mississippi River. In exchange for exclusive control of the river, Spain has promised to open ports on the Spanish mainland and its Mediterranean islands, as well as to the Canary Islands and the Azores, to American ships and their goods. The northern states, which controlled U.S. shipping and freight transportation in the United States, benefited the most from the proposed contract. In addition, many political leaders in the North believed that a closed Mississippi River would slow Western expansion, preserving Eastern land values, free supply of labour, and political domination of the seven northern states. Delegates from Virginia and North Carolina won congress, even when many northerners met in secret to discuss the constitution of a confederation. Negotiations proceeded quickly and at the end of April, the American envoys agreed to pay US$11,250,000 and accepted US$3,750,000 against France by their citizens. In return, the United States acquired the vast area of Louisiana territory, about 828,000 square miles of land. In October, Congress ratified the purchase, and in December 1803, France formally transferred authority over the region to the United States. The acquisition of the Louisiana territory at a bargaining price of less than three cents per hectare was Thomas Jefferson`s most remarkable performance as president. The American westward expansion to the new countries began immediately, and in 1804 a territorial government was created. On April 30, 1812, exactly nine years after the Louisiana Purchase Agreement, the first of 13 carved states on louisiana territory was admitted to the Union as the 18th U.S. state.
Prior to the treaty, the western and southern borders of the United States had been a source of tension between Spain and the United States. The U.S. border extended as far as the Mississippi River, but its southern section remained on Spanish territory, and Spanish officials, who refused to promote U.S. trade and settlement in a strategic border area, kept the Mississippi River closed to American shipping. In addition, both Spain and the United States have claimed part of the current states of Alabama and Mississippi, and previous negotiations on the settlement of territorial disputes have been halted to no avail. The Spanish government held several forts in the disputed areas and was able to count on indigenous resistance against American attempts to measure or cover Indian countries. U.S. citizens in southern countries and border regions found Spanish policy restrictive and wanted the U.S. government to renegotiate its positions. In 1784, the Spanish closed New Orleans to American products that descended the Mississippi. In 1795, the border was colonized and the United States and Spain entered into a trade agreement. New Orleans was reopened and the Americans could transfer goods without paying the transportation fee, the pawn, if they transferred goods from one ship to another.
 The Treaty of San Lorenzo allowed and encouraged American settlers to continue expanding westward, making border areas more attractive and lucrative. As a result, it has been popular with the American public, especially in the west and south. With Thomas Pinckney linked to the federalist party, the treaty served to strengthen federalists outside its stronghold in England and to give the party a stronger base in areas where it was traditionally weak. From a diplomatic point of view, the treaty marked a reversal of Spanish policy, which tried to maintain a strong buffer region in North America, while the United States was placed in a stronger position vis-à-vis the European powers compared to the United States.