What is the central concept of Mackinder's Heartland Theory?

The central concept of Mackinder's Heartland Theory is that control of the 'Heartland,' a vast, resource-rich area in the interior of Eurasia, is the key to controlling the world. Halford Mackinder, a British geographer, introduced this theory in his 1904 paper, 'The Geographical Pivot of History.' The Heartland, according to Mackinder, is largely invulnerable to sea power because it is a landlocked region inaccessible by naval forces. Specifically, it encompasses much of present-day Russia, Central Asia, and parts of Eastern Europe. Mackinder argued that with the advent of railways, land power would become more important than sea power because railways allowed for rapid deployment of military forces across vast land areas, a capability that sea power lacked in relation to the Heartland. The theory states that: 'Who rules East Europe commands the Heartland; Who rules the Heartland commands the World-Island; Who rules the World-Island commands the world.' The 'World-Island' refers to Eurasia and Africa combined, which Mackinder considered the largest and most populous landmass on Earth. Mackinder believed that a power controlling the Heartland would have the resources and strategic position to dominate the World-Island and, ultimately, the world. Though written in the early 20th century, Mackinder's theory has had a lasting impact on geopolitical thinking, influencing policymakers and strategists throughout the 20th and 21st centuries. It shaped perceptions about the strategic importance of Central Asia and the need to prevent any single power from controlling this region. For example, the Cold War rivalry between the United States and the Soviet Union can be partly understood through the lens of Mackinder's theory, with both superpowers vying for influence in the areas surrounding the Heartland.