Before the Louisiana Purchase, Louisiana was a French territory, and Napoleon threatened to block American access to the port of New Orleans on the Mississippi River. This scared Americans, because the nation wanted to expand the exports of American products.
President Jefferson sent diplomats to France to bargain access to the River, instead, Napoleon offered the sell of Louisiana. France needed money because of the war against Britain and then, the US bought Louisiana for 15 million dollars.
The Purchase impacted the Western Expansion since it doubled the size of the US and added more 14 states to the Union. It permitted the US to have more land to plantations and increase commerce and trade.
The philosophy of “Manifest Destiny” also became popular with the belief that Americans were destined to expand their territory.