After Ukraine and Russia’s agreement to a Black Sea ceasefire as part of broader talks being brokered by the United States in Saudi Arabia, the next step is to design a coalition maritime force to secure the peace. This force will need multiple layers of defense, intelligence sharing, and the ability to counter hostile acts—particularly those aimed at grain corridors, critical infrastructure, or coastal regions. Russia must be deterred, and if necessary, prevented, from using the maritime domain to hold Ukraine at risk as part of a broader coercive negotiation posture Putin is almost certain to adopt.
Yet any maritime security arrangement must be balanced against the constraints of the Montreux Convention, which regulates naval access to the Black Sea, and against existing requirements for NATO members to guard their coastal waters and key sea lines of communication that connect Europe to the world. While these limitations complicate manning a Black Sea maritime security force, they do not preclude establishing a robust deterrent posture—one that combines Black Sea navies and NATO partners conducting persistent maritime patrol reconnaissance and is networked with larger contracted and open-source intelligence, coastal defenses, and short-term deterrent patrols surging into the region. The net result is a multidomain fleet-in-being that allows NATO to secure the peace in the Black Sea without compromising already stretched multinational naval forces.
Navigating Limitations
Since it came into being in 1936, the Montreux Convention enshrines Turkey’s control over the Bosphorus and Dardanelles straits, imposing restrictions on both tonnage and the types of warships that non–Black Sea states may deploy. During peacetime, non-littoral warships can generally remain in the Black Sea for up to 21 days, and their total tonnage is also capped. Although this framework was designed for an earlier era, it remains a crucial legal constraint on the size of any international naval presence seeking to safeguard a ceasefire in Ukraine.
In practical terms, Montreux means that large standing deployments of U.S. or other NATO surface forces face tight limits on both time and tonnage. The easiest workaround would be to rely on littoral states—particularly Bulgaria, Romania, and Turkey itself—to supply continuous surface combatant presence. Meanwhile, non-Black Sea NATO partners can contribute through scheduled deployments, rotating warships on short notice, and deterrent patrols under strict compliance with Montreux. These patrols can be augmented by a mix of maritime patrol and reconnaissance (MPRA) connected to commercial intelligence using a mix of military aircraft, contracted aircraft, and space-based collection to create a common operating picture of the Black Sea at releasable levels of classification. This multidomain common operating picture helps NATO member states connect their coastal defenses—a mix of aircraft patrols alongside surface ships all equipped with anti-submarine and anti-ship capabilities—to deter Russia from challenging the ceasefire, or worse, attacking NATO interests in the Black Sea. Furthermore, a common operating picture allows NATO planners to better balance the mix of forces required to secure the peace in the Black Sea with its existing commitments to maintain standing naval forces and four naval groups, including mine countermeasure flotillas, in the Atlantic, Baltic, and Mediterranean. This balancing act is further complicated by endemic issues with readiness across multiple NATO navies including the United States. Each ship down for emergent or planned maintenance and lifecycle modernization is one less asset available to deter Russia in the Black Sea and beyond.
This readiness challenge is further compounded by the need for European navies – along with the US and Canada – to safeguard sea lines of communication challenged by states like including China and Iran, including through proxies such as the Houthis who are attacking shipping in the Red Sea. Each warship in the Red Sea or supporting a freedom of navigation patrol in the Asia-Pacific is one less asset to deter Russia. The net result is a need to find creative solutions to deterring Russia and safeguarding the Black Sea ceasefire that do not limit the ability of NATO to play a positive role globally.
A Multidomain Fleet in Being
Connecting coastal defenses with a common operating picture creates a multidomain force in being. The term “fleet in being” was popularized by Vice Admiral Philip Colomb’s book Naval Warfare in 1891, but dates back to the late seventeenth century. The term is divisive among naval historians; it was attacked by Alfred Thayer Mahan in his biography of Nelson, but advocated by Sir Julian Corbett as the principal method for disrupting an adversary’s ability to conduct sea control (i.e., command of the sea). The concept has had a renaissance in the last ten years as thinkers in the U.S. Marine Corps and U.S. Army explore how to connect long-range missiles and battle networks to generate multidomain effects. In turn, these ideas have led to new littoral forces (e.g., Expeditionary Advanced Bases, Marine Littoral Regiment, Multidomain Task Force) alongside concepts like archipelagic defense and fortress fleets.
Applied to the Black Sea, creating a battle network that connects a mix of air (e.g., MPRA), space, and land (i.e., surveillance radar) sensors to coastal defenses and surface ship and submarine patrols by Black Sea navies, augmented periodically by 21-day guided missile frigate and destroyer deployments, creates a multidomain fleet in being. This force aligns with the principles of economy of force and security to provide NATO flexibility.
NATO would need to create a new command around this mission with Black Sea states playing a leading role. Countries like Romania, Bulgaria, and Turkey would create small task groups connected to a larger task force integrating NATO MPRA and coastal defenses. The United States would likely provide support in the form of contracted Intelligence, Surveillance, and Reconnaissance (ISR) and space-based intelligence, thus freeing up its forces to focus on the Asia-Pacific without sacrificing support to NATO mission and securing the Black Sea. This support could be augmented by periodic deterrent patrols into the Black Sea by U.S. Navy ships as they transit the Mediterranean, a practice that dates back to the Cold War. Furthermore, the United States can use other means such as foreign military sales and security cooperation to support states seeking to strengthen their anti-ship, anti-submarine, and mine countermeasures. For example, the United States could align its growing network of firms specializing in unmanned surface and subsurface vehicles with NATO member states making similar investments. These efforts have added benefit of creating a network of manufacturers among the United States and its allies that can surge production of unmanned systems for future maritime demands. The United States can also support regional coproduction initiative efforts by Romanian and Ukraine to codevelop anti-ship missiles.
This mix of force employment and development activities would support five primary missions for a NATO multidomain task force—a fleet in being—in the Black Sea.
Securing grain corridors: Ensuring safe passage for merchant vessels departing Ukrainian ports has become a strategic priority, given Russia’s willingness to harass shipping vessels or target Ukrainian maritime port’s cargo infrastructure. Positioning surface vessels to escort convoys or rapidly respond to suspicious activity would deter opportunistic strikes by Russian naval or paramilitary forces.
Monitoring ceasefire violations: Even with a negotiated end to the ground conflict, the maritime domain is ripe for spoilers. Regular patrols and maritime domain awareness efforts can document any hostile maneuvers or violations—helping to enforce accountability and build confidence in the ceasefire process. The key to this line of effort will be creating a robust constellation of contracted ISR, commercial space, and MPRA assets that enable a common operating picture releasable to Ukraine and the world to hold Russia accountable.
Coordinating with coastal defenses: Ukraine and littoral partners can integrate their surface deployments with shore-based assets such as anti-ship missile batteries and surveillance radars. This integrated posture complicates the threat calculus for Russia, reducing the likelihood of an opportunistic strike.
Mine clearance: Russia aggressively mined Ukrainian ports and retains the ability to lay additional mines if the ceasefire breaks down. Two years into the war, NATO naval forces in the Black Sea were still discovering new mines. As a result, the task force will need to include naval mine countermeasure vessels and capabilities linked to the NATO center for excellence.
Deterrent patrols: NATO member states, including the US, will need to periodically surge in surface combatants that comply with the Montreux Convention in order to augment the multidomain fleet in being. These deterrent patrols should be short notice to maximize surprise and keep the Russian Black Sea Fleet focused more on sea denial and defense than sea control. By using short notice patrols, NATO also limits the impact on its fleet readiness. Just as important, these surges should be accompanied by other domain activity such as the use of cyber protection teams to practice defending port infrastructure in the Black Sea.
Securing the Ceasefire Requires a Multidomain Fleet in Being for the Black Sea
In the long run, successfully securing the Black Sea during a Ukraine-Russia ceasefire requires balancing deterrence with constrained resources and legal obligations. A multidomain fleet in being that integrates littoral navies, rotational NATO deployments, contracted ISR, and robust coastal defenses offers a flexible, cost-effective solution. By focusing on key tasks like protecting grain corridors, monitoring ceasefire compliance, and sustaining readiness through short-notice deterrent patrols, NATO can address Russia’s threats without compromising global naval commitments. Although the Montreux Convention limits large sustained deployments by non–Black Sea states, the alliance can still establish credible maritime security through smaller rotating forces and advanced intelligence sharing networks. Equally important, the new task force offers a forum for better integrating Turkey into NATO operations.
Ultimately, forging this hybrid maritime approach will help maintain stability, reinforce the ceasefire, and uphold the principle that aggression in the Black Sea remains off-limits—both now and for the future. Just as important, it provides a testbed for developing novel maritime solutions that can be applied elsewhere. For example, Taiwan should be invited to observe and draw insights as it seeks to generate options for its own maritime territorial defense. Securing the Black Sea during the ceasefire provides a testbed for new approaches to sea denial required in an era of global competition.
Benjamin Jensen is director of the Futures Lab at the Center for Strategic and International Studies (CSIS) in Washington, D.C. Mark Montgomery is a retired rear admiral with the U.S. Navy and senior director of the Center on Cyber and Technology Innovation at the Foundation for the Defense of Democracies.