The Territory of Colorado was an organized incorporated territory of the United States that existed from February 28, 1861, until August 1, 1876, when it was admitted to the Union as the 38th State of Colorado. The territory was created during the onset of the American Civil War in the wake of the Pike's Peak Gold Rush, which brought the first large concentration of settlers from the east to the region. The war stifled growth, but following the war mining grew more industrialized and fueled the economy. The construction of railroads further improved the economy. The boundaries of the newly designated Colorado Territory were very similar to those of the modern State of Colorado.