The **Louisiana Purchase** in **1803** significantly impacted American expansionist policies by effectively doubling the size of the United States. Acquiring land from France, it opened vast territories for exploration and settlement, fueling the **Manifest Destiny** ideology. This purchase encouraged westward migration and the quest for resources, facilitating the growth of agriculture and commerce. Additionally, it set the stage for increasing tensions regarding **slavery**, as new states were added to the Union. The acquisition strengthened the U.S. position in North America and exemplified the nation's ambitions for land and economic expansion.