Historical Phases

What happened

For its first four decades the Alliance defended a continental front in central Germany, and the geography this module describes lay beyond it. Every state east of the German border that hosts a NATO formation today was then either a member of the Warsaw Pact or a republic of the Soviet Union. The instruments that would later build the flank existed from the start — Article 5, Article 3 and, decisively, Article 10's provision for accession — but had no eastern application.

The period covers

• The treaty's twelve signatories included no state east of the later inner-German border.
• The Baltic states, Poland, Czechia, Slovakia, Hungary, Romania and Bulgaria were on the other side of the confrontation, not inside the Alliance.
• Finland and Sweden were outside the Alliance by their own policy, which is the fact the module's northern arena turns on.

Key events in this phase

What happened

The Alliance took in the states of central Europe and then the Baltic republics while simultaneously attempting a partnership with Russia. The organising logic was that enlargement and partnership were compatible, and the posture followed from it: the new members received no permanent allied ground forces. The one standing military presence the Alliance did create in the east was in the air, because the three Baltic states had no fighter force of their own.

The period covers

• Czechia, Hungary and Poland acceded in 1999; seven more states, including the three Baltic republics, in 2004.
• Baltic Air Policing began with the 2004 accession and has run continuously since, on a four-month rotation of volunteered aircraft.
• The NATO Response Force was announced at Prague in 2002 and reached initial operational capability in 2004 — a rotational, deployable force for crises, not a standing posture in the east.

Key events in this phase

What happened

Russia's annexation of Crimea ended the partnership premise, and the Alliance's answer was assurance and adaptation rather than mass. The Readiness Action Plan created a spearhead force inside the NATO Response Force and the integration units that would receive reinforcements; the Warsaw decisions turned that into a standing multinational presence — four battalion-scale battlegroups in the north-east and a tailored presence in the south-east. The logic was tripwire and reassurance: a force large enough that an attack on it is an attack on several Allies, not large enough to hold ground alone.

The period covers

• Practical cooperation with Russia was suspended in April 2014; political and military communication channels stayed open.
• Four multinational battlegroups were deployed and fully operational in Estonia, Latvia, Lithuania and Poland by August 2017.
• The south-east received a tailored presence on land, at sea and in the air rather than battlegroups, and the Black Sea response of 2018 was about situational awareness.

Key events in this phase

What happened

The full-scale invasion ended reassurance as an organising idea. Within a month Allies agreed four further battlegroups, extending forward presence from the Baltic to the Black Sea and roughly doubling the troops on the ground; within four months they had agreed to scale beyond battalion size where and when required and to build a new force model. Finland and Sweden applied. The period closes with the approval of a new generation of regional defence plans, which changed what the forces were for: not reassurance, but execution of a plan.

The period covers

• The highest-readiness element of the NATO Response Force deployed for the first time, to Romania.
• Eight battlegroups by March 2022; brigade-scale presence agreed at Madrid in June.
• Finland and Sweden applied in May 2022 and accession protocols were signed in July; Finland acceded in April 2023.

Key events in this phase

What happened

The decisions of 2022 and 2023 became structures. Sweden acceded, completing the Nordic enlargement; the NATO Force Model fully replaced the NATO Response Force and the Allied Reaction Force launched; Latvia and Lithuania became the first hosts with brigade-scale formations, one of them in a base NATO calls permanent; a land component command appeared in Finland. In parallel a second kind of pressure became institutional: after repeated damage to Baltic cables and pipelines, NATO created a standing activity to watch over undersea infrastructure.

The period covers

• Sweden acceded in March 2024; the NATO Force Model transition completed on 1 July 2024.
• Latvia scaled to a brigade in July 2024; Germany began building a permanent brigade in Lithuania from April 2024.
• Baltic Sentry launched in January 2025 after several incidents involving critical undersea infrastructure.

Key events in this phase

What happened

Russian drones and aircraft violated the airspace of seven Allies in a single month, two of them invoking Article 4 consultations, and the Alliance answered with a named, flexible, multi-domain activity spanning the whole flank rather than with a new permanent structure. The command layer moved underneath it: leadership of the strategic commands was redistributed among Allies, a bi-national corps took responsibility for two Baltic states, and the ninth and last forward battlegroup was established in Finland under Swedish framework leadership.

The period covers

• Eastern Sentry launched in September 2025, building on the Baltic Sentry experience and complementing standing air policing.
• Command-structure leadership was redistributed on 6 February 2026, to be implemented incrementally with scheduled rotations.
• 1 (German/Netherlands) Corps assumed responsibility for Estonia and Latvia formally on 30 June 2026; NATO established the ninth battlegroup in the same month.

Key events in this phase