The Geopolitics of the Caspian Sea: Deciphering the Legal Architecture of the 2018 Aktau Convention

The legal classification of the Caspian Sea has long stood as one of the most complex puzzles in modern international public law. Spanning approximately 371,000 square kilometers, this landlocked body of water lies at the intersection of Europe and Asia, boasting some of the world’s largest untapped oil and natural gas reserves, alongside a fragile and highly endemic ecosystem. For nearly three decades following the dissolution of the Soviet Union, the five littoral states—the Russian Federation, the Republic of Azerbaijan, the Republic of Kazakhstan, Turkmenistan, and the Islamic Republic of Iran—remained locked in a diplomatic stalemate over how to divide its waters, seabed, and subsoil. The signature of the Convention on the Legal Status of the Caspian Sea on August 12, 2018, in the Kazakh port city of Aktau, marked a historic turning point, establishing a highly customized legal framework designed to resolve these overlapping claims.

This article provides a comprehensive legal and geopolitical analysis of the 2018 Aktau Convention. It examines the historical treaties that governed the Caspian prior to 1991, deconstructs the conceptual debate regarding its status as a sea or a lake, and scrutinizes the innovative solutions codified in the treaty. Furthermore, it evaluates the treaty’s security provisions, the mechanics of seabed delimitation, and the environmental obligations that shape regional pipeline politics in an increasingly fragmented Eurasian landscape.

Historical Foundations and the Post-Soviet Geopolitical Shock

To understand the complexity of the Aktau Convention, one must first examine the historical legal regime that governed the Caspian prior to the collapse of the Soviet Union. For the greater part of the nineteenth and twentieth centuries, the Caspian was effectively a bilateral space shared between the Russian Empire (later the USSR) and Persia (later Iran). This relationship was regulated by a series of bilateral agreements, most notably the 1921 Treaty of Friendship between the Russian Socialist Federative Soviet Republic and Persia, and the 1940 Treaty of Commerce and Navigation between the Soviet Union and Iran.

These treaties established a legal regime of condominium—a system of joint ownership and shared utilization. Under the 1921 and 1940 agreements, the Caspian was defined as a joint Soviet-Iranian water body, closed to non-littoral states. Both nations enjoyed freedom of navigation throughout the entire sea, with the exception of a ten-nautical-mile exclusive fishing zone adjacent to their respective coastlines. Crucially, these historic agreements left the seabed and its subsoil entirely undivided. At the time, offshore extraction technology was in its infancy, and neither state anticipated the massive subsea hydrocarbon boom that would define the late twentieth century.

The geopolitical landscape shattered in December 1991. With the dissolution of the Soviet Union, the number of Caspian littoral states expanded from two to five. While the newly independent sovereign states of Azerbaijan, Kazakhstan, and Turkmenistan initially acknowledged their treaty obligations under the 1991 Almaty Declaration, they quickly recognized that the outdated Soviet-Iranian legal framework was wholly inadequate for their developmental needs. These young states possessed vast, underexploited offshore oil and gas fields but lacked the legal clarity required to attract the foreign direct investment necessary for deepwater exploration. Thus began a twenty-seven-year diplomatic odyssey to draft a new constitution for the Caspian Sea.

Is the Caspian Sea legally a sea or a lake?

At the heart of the decades-long diplomatic impasse lay a fundamental question of international law: was the Caspian Sea legally a sea or a lake? The answer to this question was not merely academic; it carried profound legal, economic, and security consequences, dictating which bodies of international law would govern the distribution of the Caspian’s resources.

If the Caspian were classified as a sea, the 1982 United Nations Convention on the Law of the Sea (UNCLOS) would apply. Under UNCLOS, coastal states are entitled to a twelve-nautical-mile territorial sea, a contiguous zone, and an Exclusive Economic Zone (EEZ) extending up to 200 nautical miles, alongside rights over the continental shelf. Because the Caspian is relatively narrow, the application of UNCLOS would require the drawing of median lines to divide the entire body of water—both the water column and the seabed—into sovereign national sectors. This option was highly favored by Kazakhstan and Azerbaijan. Kazakhstan, possessing the longest coastline, stood to gain a massive portion of the Caspian’s northern waters, which contain rich hydrocarbon deposits like the Kashagan field.

Conversely, if the Caspian were classified as a lake, it would be governed by the international law of transboundary lakes and international rivers. In international practice, transboundary lakes are not subject to UNCLOS. Instead, they are typically regulated either by the principle of res communis (shared common property where any activity requires the consensus of all littoral states) or partitioned into equal, negotiated sectors. The Islamic Republic of Iran strongly advocated for this approach. Boasting the shortest and most concave coastline (accounting for approximately thirteen to fourteen percent of the total), Iran would receive a relatively small portion of the Caspian under an UNCLOS-style median line division. Consequently, Tehran argued for an equal twenty-percent division of both the seabed and the surface waters among the five states, or a continuation of the historic condominium regime.

The Caspian Sea, however, defies easy classification. Geographically, it is the world’s largest landlocked water body, lacking a natural, direct outlet to the global ocean system—a key characteristic of seas under international law. Yet, its massive size, salinity, and oceanic basin structure make treating it as a simple freshwater lake legally and practically untenable. The five states eventually realized that forcing the Caspian into a rigid, pre-existing international legal category would never yield a consensus. The solution required an entirely original, custom-tailored framework.

What is the legal status of the Caspian Sea under the 2018 Aktau Convention?

The signing of the Aktau Convention in 2018 resolved this gridlock by establishing a unique, sui generis (of its own kind) legal status for the Caspian Sea. Rather than designating the Caspian as either a sea or a lake, the Convention created a hybrid legal regime that treats the water column and the seabed as separate legal entities, blending elements of the law of the sea with custom regional compromises.

Under the terms of the Aktau Convention, the water column is divided into distinct zones of national jurisdiction, while the remaining central portion remains shared. The Convention outlines three primary maritime zones:

• Territorial Waters: Each littoral state is granted sovereignty over a belt of water extending up to fifteen nautical miles from its coastal baselines. The outer limit of these territorial waters constitutes the state’s maritime border.
• Fishery Zones: Adjacent to its territorial waters, each state possesses a ten-nautical-mile exclusive fishery zone. Within this zone, the coastal state holds sole rights to harvest biological resources, a critical provision for managing the Caspian’s famous, yet endangered, sturgeon populations.
• Common Maritime Space: Beyond these twenty-five nautical miles of combined territorial and fishery zones lies the common maritime space. This central area remains open to the navigation of all littoral states, preserving the spirit of joint access found in the historic Soviet-Iranian treaties.

This tiered structure represents a major compromise. By establishing territorial waters and fishery zones, the states secured the sovereign security and resource protection they desired, resembling the zoning principles of UNCLOS. By designating the central waters as a common maritime space, they preserved the freedom of transit and shipping that is vital for regional trade, satisfying those who favored a collaborative, lake-style regime.

Can non-Caspian states deploy military forces in the Caspian Sea?

Beyond resource allocation, the Aktau Convention served as a crucial security instrument, addressing the geopolitical anxieties of the region’s dominant military powers. A primary objective for both the Russian Federation and Iran was to prevent external military actors—most notably the United States and member states of the North Atlantic Treaty Organization (NATO)—from gaining a strategic foothold in the Caspian basin.

This objective was successfully codified in Article 3 of the Convention, which outlines the fundamental principles governing the activities of the littoral states. Specifically, Article 3(11) establishes the principle of "the non-presence in the Caspian Sea of armed forces not belonging to the Parties." Furthermore, Article 3(12) prohibits the provision of a littoral state’s territory to any third state for the purpose of committing aggression or launching military operations against another Caspian state.

These security clauses represent a major diplomatic victory for Moscow. For Russia, the Caspian Sea serves as a critical strategic asset and a secure "inner lake" for military operations. During the Syrian conflict, Russia repeatedly demonstrated its ability to project power from the region by launching Kalibr cruise missiles from its Caspian Flotilla vessels against targets in Syria. By legally barring non-littoral military forces, the Convention ensures that Russia faces no naval competition from Western powers within this landlocked basin.

For Azerbaijan and Kazakhstan, both of which have historically maintained security cooperation and joint military exercises with Western nations, accepting these military restrictions was a necessary concession. In exchange for limiting their foreign defense partnerships within the Caspian waters, Baku and Nur-Sultan (now Astana) secured Russian recognition of their sovereign economic rights over their respective offshore oil and gas sectors.

How is the seabed of the Caspian Sea divided?

The physical division of the Caspian’s rich subsea resources was the most economically contentious issue during the drafting process. The Aktau Convention resolved this by establishing a clear distinction between the water column and the seabed. While the water column in the center remains shared as a common maritime space, the seabed and its subsoil are entirely partitioned into national sectors.

Article 8 of the Convention dictates that "the delimitation of the Caspian Sea seabed and subsoil into sectors shall be effected by agreement between adjacent and opposite States, in accordance with generally recognized principles and rules of international law." This wording is highly significant because it does not impose a single, uniform formula for dividing the entire Caspian seabed. Instead, it delegates the responsibility of delimitation to the specific neighboring states, allowing them to negotiate bilateral and trilateral agreements that suit their unique geographical realities.

This approach legally validated a series of bilateral agreements that had already been concluded in the northern and central sectors of the Caspian. Long before the Aktau summit, several states had bypassed the overall diplomatic deadlock by resolving their boundaries bilaterally:

Parties involved	Year of Agreement	Methodology and Purpose
Russia & Kazakhstan	1998 (Protocol in 2002)	Divided the northern seabed using a Modified Median Line (MML) while keeping the water column shared for navigation.
Kazakhstan & Azerbaijan	2001 (Protocol in 2003)	Established maritime boundaries in the central-northern sector based on the MML, delineating sovereign hydrocarbon sectors.
Russia & Azerbaijan	2002	Delimited adjacent sectors in the western Caspian along the MML, securing clear lines for oil and gas exploration.
Russia, Kazakhstan & Azerbaijan	2003 (Trilateral)	Established the junction point of their respective seabed sectors, fully partitioning the northern half of the Caspian Sea.

While the northern and central sectors of the Caspian are fully delimited, the southern sector remains a source of diplomatic friction. The maritime boundaries between Azerbaijan, Iran, and Turkmenistan are still unresolved. Iran continues to seek a larger share than what a strict geographical median line would offer, and negotiations regarding disputed offshore oil fields have occasionally escalated into naval standoffs. However, the signing of a joint development agreement in 2021 between Azerbaijan and Turkmenistan over the long-disputed "Dostluk" (Friendship) oil and gas field indicates that the flexible, bilateral framework promoted by the Aktau Convention is successfully facilitating peaceful resolutions.

Environmental Governance and the Geopolitics of Subsea Pipelines

Beyond boundaries and military blockades, the Aktau Convention addresses one of the most pressing environmental and geopolitical issues in Central Asia: the construction of trans-Caspian subsea pipelines. For decades, Turkmenistan, which holds some of the world’s largest natural gas reserves, has sought to construct the Trans-Caspian Gas Pipeline (TCP) to transport its gas across the seabed to Azerbaijan, where it could connect to the Southern Gas Corridor and enter the lucrative European market. Historically, Russia and Iran blocked this project, arguing that any major infrastructure project in the Caspian required the unanimous consent of all five littoral states.

Article 14 of the Aktau Convention introduced a major legal shift. It states that "the Parties may lay submarine cables and pipelines on the bed of the Caspian Sea." Crucially, Article 14(2) specifies that the determination of the route for laying such cables and pipelines "is subject to agreement only with the Party through whose sector the cable or pipeline is to be run." This represents a significant victory for Turkmenistan and Azerbaijan, as it theoretically strips Russia and Iran of a direct veto over the Trans-Caspian Pipeline.

However, this right is not absolute. Under Article 14(2), the construction of subsea pipelines must comply with the environmental standards and requirements set forth in international agreements to which the states are party. Chief among these is the 2003 Framework Convention for the Protection of the Marine Environment of the Caspian Sea, commonly known as the Tehran Convention. The Tehran Convention, along with its subsequent protocols—most notably the 2018 Moscow Protocol on Environmental Impact Assessment in a Transboundary Context—mandates that any project likely to have a significant transboundary environmental impact must undergo rigorous assessment and consultation with all affected littoral states.

This environmental clause creates a complex legal gray zone. While Russia and Iran no longer possess a direct sovereignty-based veto over subsea pipelines, they can still utilize the environmental assessment protocols of the Tehran Convention to delay, challenge, or block pipeline projects like the TCP. Given the Caspian’s unique status as an enclosed basin with an incredibly fragile ecosystem, environmental concerns are highly valid; however, in the highly competitive Eurasian energy market, environmental arguments are frequently utilized as geopolitical instruments to protect national energy monopolies.

Conclusion: The Future of the Caspian Legal Order

The 2018 Aktau Convention stands as a major achievement in the history of international diplomacy and regional security. By devising a highly innovative, sui generis legal regime, the five littoral states successfully resolved a decades-old dispute that threatened regional stability and hindered economic development. The treaty managed to balance the competing demands of the coastal states: granting them sovereign rights over local resources and subsea sectors while maintaining shared access to central waters and strictly preventing the intrusion of external military powers.

Nevertheless, the legal architecture of the Caspian Sea remains a work in progress. As of today, the Convention has not yet entered into full legal force because the Islamic Republic of Iran has delayed its final legislative ratification. Within Iran, the treaty has faced significant domestic political scrutiny, with critics arguing that the country conceded too much of its historic condominium rights without securing a guaranteed twenty-percent share of the seabed. Furthermore, the ongoing geopolitical shifts in Eurasia—most notably the escalation of Western sanctions against Russia and Iran following the 2022 invasion of Ukraine—have altered the strategic calculation of the littoral states.

As Russia seeks new trade corridors to the Global South via the International North-South Transport Corridor (INSTC), and as European nations actively look to Central Asia for alternative energy resources through the Middle Corridor, the Caspian Sea is once again at the center of global geopolitics. Whether the Aktau Convention can withstand these mounting geopolitical pressures remains to be seen, but its innovative legal framework will undoubtedly remain the cornerstone of Caspian diplomacy and international maritime law for decades to come.