Final answer:
The concept of Manifest Destiny was a driving force behind the expansion of the territory west of the Mississippi River from 1820 to 1860, supplemented by economic motivations such as the California Gold Rush and the desire for fertile farming land.
Step-by-step explanation:
One specific force that propelled the development of the territory west of the Mississippi River during the period 1820 to 1860 was the concept of Manifest Destiny. This ideology held that Americans were destined to expand their democratic institutions across the continental United States. A combination of demographic growth, economic opportunities such as the California Gold Rush of 1849, and political incentives like the desire to secure land before foreign powers, all contributed to the westward expansion movement. Additionally, economic motivations played a pivotal role, including the desire for new farming land due to soil depletion in the East and the pursuit of new trade opportunities with Asia by establishing control over west coast ports.