Horses were likely confined primarily to the Sydney region until the early 19th century, when settlers first crossed the Blue Mountains and opened expansion inland. Horses were required for travel, and for cattle and sheep droving as the pastoral industry grew. The first report of an escaped horse is in 1804, and by the 1840s some horses had escaped from settled regions of Australia. It is likely that some escaped because fences were not properly installed, when fences existed at all, but it is believed that most Australian horses became feral because they were released into the wild and left to fend for themselves. This may have been the result of pastoralists abandoning their settlements, and thus their horses, due to the arid conditions and unfamiliar land that combined to make farming in Australia especially difficult. After World War I, the demand for horses by defence forces declined with the growth in mechanization, which led to a growth in the number of unwanted animals that were often set free. Throughout the 20th century, the replacement of horses with machines in farming led to further reductions in demand, and may have also contributed to increases in feral populations.