According to Mackinder, the Earth's land surface was divisible into:
The Heartland lay at the center of the world island, stretching from the Volga to the Yangtze and from the Himalayas to the Arctic. Mackinder's Heartland was the area then ruled by the Russian Empire and after that by the Soviet Union, minus the Kamchatka Peninsula region, which is located in the easternmost part of Russia, near the Aleutian Islands and Kurile islands.
Later, in 1919, Mackinder summarized his theory as:
"Who rules East Europe commands the Heartland; who rules the Heartland commands the World-Island; who rules the World-Island commands the world." (Mackinder, Democratic Ideals and Reality, p. 150)
Any power which controlled the World-Island would control well over 50% of the world's resources. The Heartland's size and central position made it the key to controlling the World-Island.
The vital question was how to secure control for the Heartland. This question may seem pointless, since in 1904 the Russian Empire had ruled most of the area from the Volga to Eastern Siberia for centuries. But throughout the nineteenth century:
Mackinder held that effective political domination of the Heartland by a single power had been unattainable in the past because:
He outlined the following ways in which the Heartland might become a springboard for global domination in the twentieth century (Sempa, 2000):
The combined empire's large East Asian coastline would also provide the potential for it to become a major sea power. Mackinder's "Who rules East Europe commands the Heartland ..." does not cover this scenario, probably because the previous 2 scenarios were seen as the major risks of the nineteenth century and the early 1900s.
One of Mackinder's personal objectives was to warn Britain that its traditional reliance on sea power would become a weakness as improved land transport opened up the Heartland for invasion and / or industrialization (Sempa, 2000)
Also see Rimland Theory, Eurasia, Sea theory.