In the 21st century, with China’s rise and the United States(U.S.)’ relative decline, the U.S.-China relationship together with strategic environment in Northeast Asia is changing. China is expanding its influence from Northeast Asia to Southeast Asia and Central Asia. As a result, China is forming a new type of ‘check and balance,’ and ‘cooperation and conflict’ with the U.S. in Southeast Asia and Russia in Central Asia. Such changes in China’s foreign relations policy and strategy as well as the strategic environment in Northeast Asia will have a higher possibility of influencing the unification on the Korean Peninsula. Therefore, it is necessary to precisely analyze the international situation surrounding the Korean Peninsula and to examine China’s foreign relations with countries in Northeast Asia, Central Asia and Southeast Asia. This research provides the analysis of China’s external relations policy in these three regions to understand the international circumstances surrounding the Korean Peninsula. The research aims to form the basis for the future unification diplomatic strategy. The first part of this book explains China’s major diplomatic relationships towards four East Asian countries: Japan, Russia, North Korea, and Taiwan. This chapter particularly provides a keen analysis of developments of each relationship and discusses how China initiates its diplomatic strategy and embodies its four major diplomatic relationships. Next part analyzes China’s aggressive foreign policy in Central Asia. This part suggests that China’s active policies towards Central Asian countries during the past decade is due to the increased importance of the region to China for the following aspects: national security, geopolitical location, energy and market. Considering the aforementioned factors, China’s enhanced influence in the region and intensified relations with the neighboring countries are mainly examined. The last part explores Southeast Asia’s foreign policy in the context of the rise of the G2 rivalry, focusing on Cambodia, Vietnam, and Myanmar. This part emphasizes the importance of Southeast Asia for the U.S. and China from a geopolitical perspective. Then, it goes on how each nation formed and developed its relationship with the U.S. and China, what each country’s national interest is, how they influence their foreign polices toward the two big giants, and how each country shape their policies in response to the conflictual and suspicious relationship between Washington and Beijing. In sum, this study provides an in-depth analysis of China’s overall diplomatic strategy. It is particularly significant for the policy-makers in South Korea and other countries to understand China’s external relations in the transitional situation on the East Asian regional level as well as on the global level. Keywords: Changes in East Asia’s Strategic Environment, Strategy of Unification Diplomacy, China and Southeast Asia Relations, China and Central Asia Relations. Introduction Part 1. Northeast Asia and China’s External Relations Ⅰ. Xi Jinping Administration’s Policy towards Japan Ⅱ. China-Russia Relations Ⅲ. North Korea-China Relations at a Transition Ⅳ. China-Taiwan Relations Part 2. Central Asia and China’s External Relations Ⅴ. China’s Policy towards Central Asia under the Xi Jinping Leadership Ⅵ. Central Asian States’ Responses to China’s Expansion of Power Part 3. Southeast Asia and China’s External Relations Ⅶ. Cambodia’s Relations with China and the U.S.: Norms, Interests, and a Balancing Act Ⅷ. Conflict Management and Peace Making in Myanmar: Effort and Its Effects Ⅸ. Myanmar’s Reforms and Opening the New Chapter with ASEAN Ⅹ. Learning from the Past: Vietnamese Foreign Policy in a Changing World