Until the arrival of European and American settlers in the late nineteenth century, the southern Plains of the United States were predominantly grasslands, the home and hunting grounds of many Native American tribes and the range of untold millions of bison. It was seldom used for farming. Bitterly cold winters, hot summers, high winds and especially low, unreliable precipitation made it unsuitable for standard agriculture. But at the start of the 1900s, offers of cheap public land attracted farmers to the region, and in World War I, in the midst of a relatively wet period, a lucrative new wheat market opened up. Advances in gasoline-powered farm machinery made production faster and easier than ever. During the 1920s, millions of acres of grasslands across the Plains were converted into wheat fields at an unprecedented rate.