Geopolitics, as one of the most important branches of political geography, plays a fundamental role in explaining the behavior of great powers in the international system. Classical geopolitical theories such as McKinley's Heart of the Earth theory, Spikeman's Land Margin theory, and Mahan's Sea Power theory have provided fundamental frameworks for understanding power relations in the era of geographical competition. With the world entering the 21st century and the emergence of new powers such as China, the revival of Russia's role in the peripheral environments, the continued global influence of the United States, and the transformation of Europe's position, the importance of these theories has once again become the focus of researchers' attention. In addition, new geopolitical theories and critical approaches emphasizing technology, the global economy, cyberspace, and communication networks have opened new horizons in analyzing the behavior of great powers. This article, using a comparative approach, examines the applicability of classical and contemporary geopolitical theories to explaining the behavior of major actors in the international arena. To this end, the theoretical and conceptual foundations of geopolitics are first analyzed, then each theory is evaluated in terms of its assumptions, functions, and limitations. The behavior of the United States, China, Russia, and other major players is then analyzed within the framework of these theories. The results show that many classical concepts remain evident in global politics, but new factors such as technological power, economic dependencies, energy, and cybergeopolitics indicate the need to update previous interpretations. Finally, the article emphasizes that combining traditional and modern approaches can provide a more comprehensive picture of the behavior of great powers in the modern era.