During what period of time did many western states enter the union?

Many western states entered the Union primarily during the latter half of the 19th century, particularly from the 1850s to the 1890s. This period was marked by significant westward expansion, driven by events like the California Gold Rush and the completion of the Transcontinental Railroad. States such as California, Nevada, and Colorado became part of the Union during this time, reflecting the growing American presence in the West. The process continued into the early 20th century as more territories achieved statehood.