The first wave of westward expansion accompanied the rise of manufacturing in New England and increasing mobility throughout the nation. As settlers moved to what is now the Midwest, the national infrastructure grew up around them, connecting the nation's cities and towns through a system of roads, canals and railroads. Accompanying the rise in new methods of transportation came progress in the fields of agriculture and medicine, as new machines were invented and new treatments for disease discovered. American culture developed in the form of writing, acting, and painting, and American intellectuals gained worldwide respect. Many painters and writers cited the American West as their inspiration, and the West began to symbolize the American identity: rough and rugged individualism willing to face new challenges.
However, expansion did not occur exclusively in an atmosphere of progress. The age of Jacksonian Democracy saw the rise of political strife between the ruling Democrats and the opposition Whigs. As the two-party system matured, political tension became clearly focused around the issue of slavery. As the West gradually developed, the existing states were rapidly torn apart. Economic and social divisions became accentuated and both North and South clung to their beliefs and customs.
In 1848, the Mexican War concluded, and the United States gained full control of the Texas, California and New Mexico territories. As settlers poured into these regions, it was clear that the westward expansion was closely linked to the future of slavery. North and South focused significant energy on pursuing their political desires in regard to slavery in the settled territories of the West, and the famous Lincoln-Douglas Debates had at their core the future of slavery in the West. Despite efforts at reconciliation, most notably the Compromise of 1850, the Union was thrown into a civil war over the issue of slavery from 1861 to 1865, and western expansion slowed due to the conflict.
After the Civil War and period of Reconstruction faded, expansion began again in the late 1800s. Now western settlers were spurred onward by the development of the transcontinental railroad, a major byproduct of the period of industrialization that had begun in earnest. The expansion and immigration of the late 1800s merged with this industrialization to provoke the growth of American urban society. As the needs of industrial workers became ever more important, the national political scene became dominated by the discrepancy in needs between America's rural and urban populations, as well as the needs of the new classes created by industrialization and the abolition of slavery. By the early twentieth century, the United States consisted of 48 contiguous states stretching clear across the North American continent, and with its devastatinv defeat of the Spanish in 1898 had become a legitimate international power. US cities increasingly traded with foreign markets and the nation became involved in international politics. The economic and political evolution that had accompanied, and in part resulted from, westward expansion culminated with US involvement in World War One.