NATO's Sentry Triad: The Standing Operation Replaces the Exercise

For most of the post-Cold War period, NATO demonstrated readiness through exercises: large drills with a start date, an end date, and a press release at each end. That calendar — catalogued in full in our NATO Exercises 2026 guide — still runs, but it is no longer the only instrument doing the work. Since the start of 2025 the Alliance has added a second tool that behaves differently. The three Sentry activities — Baltic, Eastern, and Arctic — have no scheduled end. They keep allied ships, aircraft, and command staff continuously deployed across the Baltic Sea, the eastern flank, and the High North. NATO has not maintained this kind of standing, multi-theatre presence on its own territory since it faced the Warsaw Pact.

All three now sit under one doctrinal label — the enhanced vigilance activity, or eVA — and that shared category is what makes them a set rather than three separate improvisations. Each was triggered by a specific incident, each is commanded from a different headquarters, and each reaches for a different instrument. Read together, they describe how NATO has chosen to hold its northern and eastern arc.

Baltic Sentry: the template

Baltic Sentry came first and set the pattern the others follow. The trigger was the severing of the Estlink 2 power cable and several communications cables between Finland and Estonia in December 2024, attributed to the anchor-drag of the tanker Eagle S. Allies issued a declaration of solidarity on 30 December, and on 14 January 2025 NATO Secretary General Mark Rutte launched the activity at a summit of Baltic Sea allies in Helsinki, co-hosted with Finnish President Alexander Stubb and Estonian Prime Minister Kristen Michal.

The command architecture is worth noting because the later Sentries reuse it. Allied Command Operations (ACO) executes; Joint Force Command Brunssum directs; Allied Maritime Command (MARCOM) coordinates the maritime effort; and a then-new body, the NATO Maritime Centre for Security of Critical Undersea Infrastructure, serves as the knowledge hub. The Supreme Allied Commander Europe (SACEUR) at the time, US Army General Christopher Cavoli, described the aim as focused deterrence throughout the Baltic Sea.

The instrument was maritime and seabed-oriented: frigates, maritime patrol aircraft, and a small fleet of naval drones, with national surveillance assets integrated into a single picture. Its operational logic has since produced its own capability programme — Task Force X-Baltic, the autonomous-surveillance initiative that eight Baltic allies moved from experimentation toward national ownership in early 2026. The point is structural: a vigilance activity launched to patrol a sea has begun generating procurement, not just patrol schedules.

Eastern Sentry: collectivising the air

Eastern Sentry extended the model from the seabed to the air. The trigger was the incursion of at least 19 Russian drones into Polish airspace on 10 September 2025 during a strike on Ukraine — what Rutte called the largest concentration of violations of NATO airspace. Warsaw requested Article 4 consultations under the North Atlantic Treaty. Rutte announced the activity two days later, on 12 September, alongside the SACEUR, US Air Force General Alexus Grynkewich, who had taken command that July, with Admiral Pierre Vandier, Supreme Allied Commander Transformation (SACT), credited for the counter-drone innovation built into it. The deterrence architecture therefore spans two SACEURs: begun under Cavoli, continued under Grynkewich.

Contributing assets came from Britain, Denmark, France, and Germany, combining conventional air-policing capability with novel counter-drone sensors and effectors along the entire flank, from the High North to the Black Sea. The structural change is the one to hold onto. Defending one’s own airspace had been a national responsibility; Eastern Sentry placed a NATO operational frame over engagement decisions along the eastern flank, a step toward collective conduct rather than national reaction. The overflight of Estonia by three Russian MiG-31s later that month tested the arrangement directly.

This is also where the unfinished business sits. As we argued in Curated No. 40, a standing air-defence operation deters only if the authority to engage is settled in advance. Persistent presence is the easy half; pre-delegated engagement authority across thirty-two members is the half Europe has not closed. Eastern Sentry makes the architecture visible and the missing permissions layer harder to ignore.

Arctic Sentry: the thinnest leg

Arctic Sentry is the newest and, on launch, the least substantial. ACO began it on 11 February 2026, led by Joint Force Command Norfolk — NATO’s only operational headquarters on the North American continent, whose area of responsibility was extended to cover the Arctic and High North in December 2025, under US Navy Vice Admiral Douglas Perry. It coordinates with the North American Aerospace Defense Command (NORAD), US Northern Command, and US European Command.

The honest reading is that at launch Arctic Sentry was less a deployed operation than a coordinating label placed over national activity already underway — Denmark’s Arctic Endurance and Norway’s Cold Response 26 among them. Wire reporting from AP and Reuters was explicit that it was not, initially, a force-generating operation. Its political function was at least as prominent as its military one. It arrived roughly a month after US President Donald Trump’s pressure on Greenland had fractured allied confidence, and NATO framed it around Russia’s Arctic militarisation and China’s growing High North interest — a formula that let the Alliance reassert collective ownership of a region Washington had just treated as a bilateral acquisition target. The UK’s commitment to double its Norway garrison from 1,000 to 2,000 troops over three years, and Exercise Lion Protector in September, are its first force contributions of substance.

What the triad amounts to

Three observations follow.

First, the geography of command is part of the meaning. Brunssum directs the Baltic; Norfolk directs the Arctic. A North American Joint Force Command running a flank that Europeans largely man is the transatlantic link turned into an operational fact — asserted at the precise moment that link is under political strain. The choice of Norfolk for the Arctic is not administrative housekeeping.

Second, the instruments differ by flank: seabed and maritime in the Baltic, air in the east, coordination of existing exercises in the Arctic. The Arctic leg is therefore the test case for whether the eVA concept can stand up a genuine operation or whether it remains, in that theatre, a relabelling of what allies were doing anyway.

Third, the exercise calendar now feeds the operations rather than substituting for them. Cold Response 26 ran under the Arctic Sentry framework; the Finnish Defence Forces placed BALTOPS 26 under the Arctic Sentry umbrella, an unusual framing given Baltic Sentry’s existing remit. The annual drills have become the force-generation engine for a standing posture — the relationship between exercise and operation has inverted.

The unresolved problem is the cost in readiness. An exercise concludes and units reset. A vigilance activity with no end date consumes ships, aircraft, and crews continuously — and NATO is now running three at once, across the Alliance’s entire northern and eastern arc, while the same fleets and squadrons are expected to populate the 2026 exercise calendar on top. The Sentries have given NATO a way to signal deterrence without firing a shot. Whether the force structure can sustain permanent presence on three flanks indefinitely is the question the next two years will answer.

Primary sources

SHAPE, Baltic Sentry to enhance NATO’s presence in the Baltic Sea (14 January 2025) — shape.nato.int. MARCOM, NATO’s Baltic Sentry steps up patrols in the Baltic Sea (14 January 2025) — mc.nato.int. NATO, NATO launches “Eastern Sentry” to bolster posture along eastern flank (12 September 2025) — nato.int. SHAPE, Eastern Sentry to enhance NATO’s presence along its Eastern flank (12 September 2025) — shape.nato.int. JFC Norfolk, NATO JFC Norfolk leads Arctic Sentry (12 February 2026) — jfcnorfolk.nato.int. Finnish Defence Forces, The Finnish Navy Command hosted the BALTOPS 2026 planning meeting (April 2026) — maavoimat.fi. NATO Allied Command Transformation, Task Force X-Baltic — act.nato.int.