The Rimland is a concept championed by Nicholas John Spykman, professor of international relations at Yale University. To him geopolitics is the planning of the security policy of a country in terms of its geographical factors. He described the maritime fringe of a country or continent; in particular the densely populated western, southern, and eastern edges of the Eurasian continent.
He criticized Mackinder for overrating the Heartland as being of immense strategic importance due to its vast size, central geographical location and supremacy of land power rather than sea power. He assumed that the Heartland will not be a potential hub of Europe, because:
Spykman thought that the Rimland, the strip of coastal land that encircles Eurasia, is more important than the central Asian zone (the so-called Heartland) for the control of the Eurasian continent. Spykman's vision is at the base of the "containment politics" put into effect by the United States in its relation/position to the Soviet Union during the post-World War II period.
Thus, 'Heartland' appeared to him to be less important in comparison to 'Rimland.'
According to Spykman, "Who controls the Rimland rules Eurasia, who rules Eurasia controls the destinies of the world."
The Rimland, Halford Mackinder's "Inner or Marginal Crescent", was divided into three sections:
Rimland or inner crescent contains most of world's people as well as large share of world's resources. Rimland is in between Heartland and marginal seas, so it was more important than Heartland. It included Asia minor, Arabia, Iran, Afghanistan, South East Asia, China, Korea and East Siberia except Russia.
All the aforesaid countries lie in the buffer zone that is between sea power and land power.
Rimland countries were amphibian states, surrounding the Eurasian continents.
While Spykman accepts the first two as defined, he rejects the simple grouping of the Asian countries into one "monsoon land." India, the Indian Ocean littoral, and Indian culture were geographically and civilization-ally separate from the Chinese lands.
The Rimland's defining characteristic is that it is an intermediate region, lying between the heartland and the marginal sea powers. As the amphibious buffer zone between the land powers and sea powers, it must defend itself from both sides, and therein lies its fundamental security problems. Spykman's conception of the Rimland bears greater resemblance to Alfred Thayer Mahan's "debated and debatable zone" than to Mackinder's inner or marginal crescent.
The Rimland has great importance coming from its demographic weight, natural resources, and industrial development. Spykman sees this importance as the reason that the Rimland will be crucial to containing the Heartland (whereas Mackinder had believed that the Outer or Insular Crescent would be the most important factor in the Heartland's containment).
He called for the consolidation of the Rimland countries to ensure their survival during World War II. With the defeat of Germany and the emergence of the USSR, Spykman's views were embraced during the formulation of the Cold War American policy of containing communist influence.
But as the states within the Rimland had varying degree of independence, and a variety of castes, races, and culture, it did not come under the control of any single power.
Also see Heart-land theory, Eurasia, Sea theory.