During the summer of 1910 large wildfires occurred throughout the western United States, and especially in the Northern Rocky Mountains. The “Great Idaho Fires” of 1910, alone burned about three million acres (~1.2 Mha)—an area that is approximately the size of Connecticut. Multiple fires ignited and coalesced, burning in forests of northern Idaho and western Montana including parts of the Bitterroot, Cabinet, Clearwater, Coeur d'Alene, Flathead, Kaniksu, Kootenai, Lewis and Clark, Lolo, and St. Joe National Forests. The firestorm burned for days in late August of 1910 and killed 87 people, including 78 firefighters. It is believed to be the largest, although not the deadliest, wildfire complex in recorded U.S. history. Here we show that highly anomalous weather preceded the conflagration in much of the West, including the occurrence of the warmest March on record for the contiguous United States (except March 2012). While the occurrence of very high winds greatly contributed to fast spread of the wildfire, the preceding highly anomalous warm and dry condition since the spring of 1910 likely also contributed to the magnitude of this event. Improved understanding of extreme wildfire outbreaks and climatological conditions associated with them is increasingly important, as such events are increasing in frequency as global and regional warming continues.