The size of nuclear tests is increasing and bombers and spy planes are flying further north. The United States increases its activities in Greenland, while the Soviet Union opens a nuclear test site in the north. Increasing espionage leads to border and airspace violations in Finland. Norway strengthens its defensive line in the north and increases the number of troops in response to the growing threat of attack. Sweden is seen as a potential future nuclear power. It also has the fourth strongest air force in the world.