The United States' northernmost military base is Thule Air Base in Greenland (above), which was strategically established to offer the Danish colonies protection from Germany in World War II. Today, the base is used for monitoring space for defence purposes. The US Air Force Space Command's 21st Space Wing uses the military base to house its suite of space sensors.
Thule is locked in by ice nine months out of the year. In the summer, a Canadian icebreaker ship clears the base's port to allow for a rapid resupply of food, fuel, construction materials and cargo.
Greenland recently came under the spotlight after US President Donald Trump made an offer to buy it. Analysts say the country is important for the US military, as it balances a Russian and Chinese commercial and military buildup in the Arctic. It could also challenge China's dominance of the world's industrial metals and help to block Russia's renewed military ambition.