China has been extensively enhancing its influence in the Indo-China region, which has immense significance for the Indo-Pacific. The Indochina region comprises of three countries - Cambodia, Laos and Vietnam, and is also called French Indochina as it was colonised and controlled by the French during the 19th and 20th centuries. It is important to understand the geopolitical dynamics of the region, given its location on the “rimland” and its geographical contiguity with China. 
The concept of rimland was introduced by Nicholas Spykman, who was a Professor of International Relations at Yale University during the Second World War. He proposed the rimland theory as an alternative approach to the Heartland theory propounded by Halford Mackinder earlier in the early part of the 20th century. According to Spykman, "Whoever controls the rimland controls Eurasia; who rules Eurasia controls the destinies of the world.” The rimland refers to the rim of the Eurasian landmass, where both the sea powers and the land powers meet. He argued that control over the rimland is pivotal for world domination, for it has both the land and sea power, and a concentration of manpower and resources.
According to the theory, most of the recent major powers aspiring for global hegemony have risen from the rimland. The footprints of Spykman are evident in the American foreign policy during the Cold War, in the policy of containment. In fact, Spykman is known as the “godfather of containment”. The rationale behind the containment strategy of the US was to keep any other power from dominating the rimland, for its own interests and security.
Spykman in his work, “America's Strategy in World Politics”, has discussed the prospects of China becoming a major player in the Asiatic rimland. According to him, “a modern, vitalized, and militarised China of 400 million people is going to be a threat not only to Japan, but also to the position of the Western Powers in the Asiatic Mediterranean”. 
The location of Indochina in the Eurasian rimland makes it important for China’s interests in the region. The Indochina region is key to China’s national security. Geographically, China has natural frontiers like deserts and mountains in the West and seas in the East. However, to its South, along the borders with countries like Laos and Vietnam, there are no natural barriers. China's deep engagement in the Vietnam War also demonstrated the strategic importance of the Indochina region.
It is important to look at China’s contemporary engagement in the region in the areas of trade, economy, hydropower, etc. Today, China engages with the countries in the region mainly through its economic endeavours. China is the largest commercial trading partner of the ASEAN countries. It also has a Free Trade Agreement with Cambodia which came into effect from 2022. With successful experience in the development of hydropower, China engages in the hydropower project projects in the Indochina region. For example, Laos is home to 81 percent of all overseas Chinese dams in the Mekong Basin. Under the Belt and Road Initiative, China has made large investments in the Indochina region.
Besides trade and economy, China also has strategic engagements with the countries in the Indochina region. China has offered military support to Cambodia and both have strengthened their military ties. With Vietnam, China has engaged in a comprehensive strategic cooperative partnership and signed a MoU on enhancing international military cooperation. This is despite the fact that China has clashing claims over South China Sea with Vietnam.
Recently, there is a speculation that China is constructing a naval facility in Cambodia at Ream. The establishment of such a base would probably be China’s second overseas military base after Djibouti. The Ream naval base is the largest and oldest base in Cambodia, but it is underdeveloped. China and Cambodia, in 2022, jointly announced a plan to upgrade the naval facility. Its proximity to the South China Sea and location at the heart of the Indo-Pacific has sparked global concerns.
China’s establishment of a naval facility at a key location like this is instrumental to its ambitions in the region, given its location in the rimland. This holds strategic implications for countries in the region and beyond. China may use the facility for power projection, making Southeast Asia “a region of overlapping military presence”. It will also make the deployment of China’s troops in the Indian Ocean Region more sustainable. Beijing may also use this facility to strengthen its position in the South China Sea. China’s possible intention to establish a naval presence has alarmed countries like Indonesia, Philippines and Vietnam; and the lack of transparency in its engagement is causing apprehensions in the region. 
While Cambodia says that the base is being developed to enhance its defence capabilities, dependence on China for the same will jeopardise its relations with ASEAN. China’s presence, overt or not, has geopolitical, military and diplomatic implications in the immediate maritime neighbourhood and the greater Indo-Pacific region. The region, most importantly, is located at the geographical starting point of China’s power projection in the rimland. Hence, the role of the Indochina region will be crucial in China’s evolving regional strategy and in shaping the international responses.
Disclaimer: The views expressed in the article are personal.