Much of Montana's prehistory has only recently been unearthed. The abundance of fossils of large and small dinosaurs, marine reptiles, miniature horses, and giant cave bears indicates that, from 100 million to 60 million years ago, the region had a tropical climate. Beginning some 2 million years ago, however, dramatic temperature changes profoundly altered what we now call Montana. At four different times, great sheets of glacial ice moved south through Canada to cover much of the north. The last glacial retreat, about 10,000 years ago, did much to carve the state's present topographic feature. Montana's first humans probably came from across the Bering Strait; their fragmentary remains indicate a presence dating between 10,000 and 4000 BC .
The Indians encountered by Montana's first white explorers—probably French traders and trappers from Canada—arrived from the east during the 17th and 18th centuries, pushed westward into Montana by the pressure of European colonization. In January 1743, two traders, Louis-Joseph and Francois Vérendrye, crossed the Dakota plains and saw before them what they called the "shining mountains," the eastern flank of the northern Rockies. However, it was not until 1803 that the written history of Montana begins. In that year, the Louisiana Purchase gave the United States most of Montana, and the Lewis and Clark expedition, dispatched by President Thomas Jefferson in 1804 to explore the upper reaches of the Missouri River, added the rest. On 25 April 1805, accompanied by a French trapper named Toussaint Charbonneau and his Shoshoni wife, Sacagawea, Meriwether Lewis and William Clark reached the mouth of the Yellowstone River near the present-day boundary with North Dakota. Shortly thereafter, the first American trappers, traders, and settlers entered Montana.
The fur trade dominated Montana's economy until 1858, when gold was discovered near the present community of Drummond. By mid-1862, a rush of miners from the gold fields of California, Nevada, Colorado, and Idaho had descended on the state. The temporary gold boom brought not only the state's first substantial white population but also an increased demand for government. In 1863, the eastern and western sectors of Montana were joined as part of Idaho Territory, which, in turn, was divided along the Bitterroot Mountains to form the present boundary between the two states. On 26 May 1864, President Abraham Lincoln signed the Organic Act, which created Montana Territory.
The territorial period was one of rapid and profound change. By the time Montana became a state on 8 November 1889, the remnants of Montana's Indian culture had been largely confined to federal reservations. A key event in this transformation was the Battle of the Little Big Horn River on 25 June 1876, when Lieutenant Colonel George Custer and his 7th US Cavalry regiment of fewer than 700 men were overwhelmed as they attacked an encampment of 15,000 Sioux and Northern Cheyenne led by Crazy Horse and Chief Gall. The following year, after a four-month running battle that traversed most of the state of Montana, Chief Joseph of the Nez Percé tribe surrendered to federal forces, signaling the end of organized Indian resistance.
As the Indian threat subsided, stockmen wasted little time in putting the seemingly limitless open range to use. By 1866, Nelson Story had driven the first longhorns up from Texas, and by the mid-1870s, sheep had also made a significant appearance on the open range. In 1886, at the peak of the open-range boom, approximately 664,000 head of cattle and nearly a million sheep grazed Montana's rangeland. Disaster struck during the "hard winter" of 1886/87, however, when perhaps as many as 362,000 head of cattle starved trying to find the scant forage covered by snow and ice. That winter marked the end of a cattle frontier based on the "free grass" of the open range and taught the stockmen the value of a secure winter feed supply.
Construction of Montana's railroad system between 1880 and 1909 breathed new life into mining as well as the livestock industry. Moreover, the railroads created a new network of market centers at Great Falls, Billings, Bozeman, Missoula, and Havre. By 1890, the Butte copper pits were producing more than 40% of the nation's copper requirements. The struggle to gain financial control of the enormous mineral wealth of Butte Hill led to the "War of the Copper Kings," in which the Amalgamated Copper Co., in conjunction with Standard Oil gave up its copper holdings. The new company, Anaconda Copper Mining, virtually controlled the press, politics, and governmental processes of Montana until changes in the structure of the international copper market and the diversification of Montana's economy in the 1940s and 1950s reduced the company's power. Anaconda Copper was absorbed by the Atlantic Richfield Co. in 1976, and the name was changed to Anaconda Minerals in 1982.
The railroads also brought an invasion of agricultural homesteaders. Montana's population surged from 243,329 in 1900 to 548,889 by 1920, while the number of farms and ranches increased form 13,000 to 57,000. Drought and a sharp drop in wheat prices after World War I brought an end to the homestead boom. By 1926, half of Montana's commercial banks had failed. Conditions worsened with the drought and depression of the early 1930s, until the New Deal—enormously popular in Montana—helped revive farming and silver mining and financed irrigation and other public works projects.
The decades after World War II saw moderate growth in Montana's population, economy, and social services. Although manufacturing developed slowly, the state's fossil fuels industry grew rapidly during the national energy crisis of the 1970s. However, production of coal, crude oil, and natural gas leveled off after the crisis and even declined in the early 1980s.
In 1983 the Anaconda Copper Mining Company shut down its mining operations in Butte. Farm income also suffered in the 1980s as a result of falling prices, drought, and insect damage. Growth in manufacturing and construction and recovery in the agricultural sector improved Montana's economy in the 1990s. However, even in the midst of a sustained economic boom, the state had the 8th-highest unemployment rate in the nation, 5.2% as of 1999. Other indicators also showed the state was not benefiting from the sustained national economic expansion of recent years. Montana faced a $230 million budget deficit in 2003, but lawmakers were able to balance the budget with a series of program reductions, new taxes, and budget transfers.
Tourism, air quality, and wildlife in parts of Montana were affected by the 1988 forest fires that burned for almost three months in Yellowstone National Park. Some Montana residents had to be evacuated from their homes. The state was among those afflicted by raging wildfires the summer of 2000, the worst fire season in more than a decade. In 2001, 2002 and 2003, wildfires broke out in the west once again. In the summer of 2002, wildfires burned over 7.1 million acres of public and private land in the US, most of it in the west. By August 2003, 36 wildfires had destroyed over 400,000 acres in Montana, equivalent to half the state of Rhode Island. Both Yellowstone and Glacier National Parks had to close sections of the parks due to fires.
In 1992 Montana's delegation to the US House of Representatives was reduced from two members to one, based on the results of the 1990 Census. The state remains one of the least populated in the nation, with an estimated 902,195 residents in 2000—or about six people per square mile.