1. Native American History: On May 15, 1868, the Fort Laramie Treaty was signed, ending Red Cloud's War between the United States and several Indigenous tribes. The treaty recognized the Black Hills as part of the Great Sioux Reservation, but it was later violated when gold was discovered in the hills, leading to the Black Hills War.

2. Environmental History: On May 15, 2017, the Yellowstone National Park's Steamboat Geyser erupted for the first time in over eight years. The geyser, which is the world's tallest active geyser, erupted several more times in the following months, leading to speculation about the causes of the increased activity.

3. Political History: On May 15, 1890, Wyoming became the first state to grant women the right to vote. Wyoming's statehood was still several months away, but the territory's leaders believed that women's suffrage would help attract more settlers to the region.

4. Cultural History: On May 15, 1876, the Deadwood Stagecoach made its first run from Cheyenne to Deadwood, South Dakota. The stagecoach was operated by the Cheyenne and Black Hills Stage Company, which was owned by Philip Danforth Armour and two partners.

5. Military History: On May 15, 1942, the Japanese launched a balloon bomb attack on Wyoming's Bairoil oilfield. The bomb, which was carried by a balloon, exploded over the oilfield, causing damage but no casualties. The attack was part of a larger effort by the Japanese to carry out attacks on the United States using balloons.