Cold-weather warfare, also known as arctic warfare or winter warfare, encompasses military operations affected by snow, ice, thawing conditions or cold, both on land and at sea. Cold-weather conditions occur year-round at high elevation or at high latitudes, and elsewhere materialise seasonally during the winter period. Mountain warfare often takes place in cold weather or on terrain that is affected by ice and snow, such as the Alps and the Himalayas. Historically, most such operations have been during winter in the Northern Hemisphere. Some have occurred above the Arctic Circle where snow, ice and cold may occur throughout the year. At times, cold or its aftermath—thaw—has been a decisive factor in the failure of a campaign, as with the French invasion of Russia in 1812, the Soviet invasion of Finland in 1939, and the German invasion of the Soviet Union during World War II.