One dramatic feature of 19th century U.S. history has been the tremendous addition of land in the West. Given the speed at which these new expanses have been added, one has to wonder why this did not happen earlier. Guillaume Vandenbroucke argues that two factors needed first to be in place: low transportation costs and a critical mass of population.
Indeed, the North-American continent is gigantic, and moving people and resources to the new lands needed new transportation technologies to be developed. These lands needed to become accessible, both for immigrants in and for products out. The standard rationale for the westward expansion, the fact that land was basically free, is in fact flawed: Before being cultivable, land needs to be prepared: clearing, fencing, irrigating, draining all sum up to considerable investments. This work needed a lot of manpower, more than was typically living locally. You thus needed transportation and surplus population to bring in.