The U.S. Goes Postal: Mapping Union and War in the Antebellum Era is a digital mapping project that tracks the growth of post offices and postal routes in the United States from 1778 to 1878. This digital map shows how the postal map expanded unevenly beyond and within the nation’s borders throughout this period. Using archival materials from the Smithsonian Postal Museum, the National Archives, and the Library Company of Philadelphia, this project depicts the communications history of the United States with unprecedented specificity. These datasets show the postal service to be a tool of colonization; post offices and routes crept into territory outside of the United States, making it easier for states like Maine, Florida, and Missouri to be integrated into the national body. Despite the postal system’s commitment to political neutrality in DC, building new post offices and sending letters westward and southward were far from neutral acts. In mapping the uneven and contentious national border, my project will show how frontier post offices were fundamental to the project of manifest destiny.