The document provides an overview of Key Concept 4.3 from the APUSH review which discusses how US foreign policy and private initiatives in the early 19th century were shaped by expanding trade and national borders. It summarizes events like the Lewis and Clark expedition, conflicts with Native Americans during the Indian Removal period, and the Monroe Doctrine. It also examines how westward expansion increased tensions between the North and South over the spread of slavery into new territories, as seen in events like the Missouri Compromise.