The Russian Foreign Ministry summoned the Finnish ambassador to Moscow on 19 December, where he was informed that the Russian side "would not leave unanswered" the NATO military build-up on the border with the Russian Federation.

Maria Zakharova, the official representative of the Russian Foreign Ministry, stated that the agreement on defence cooperation between Finland and the United States "threatens the security of the Russian Federation," so Russia will take "necessary measures to counter the aggressive decisions of Finland and its NATO allies."

She said the Finnish authorities would bear responsibility for "turning the zone of good neighbourliness in this region into a zone of possible confrontation."

On 18 December in Washington, Antti Häkkänen, Defence Minister of Finland, along with US Secretary of State Antony Blinken, signed a bilateral agreement on defence cooperation – the Defence Cooperation Agreement.

The agreement simplifies military cooperation and training – after signing it, the movement of troops and equipment will not require approvals at the same high level as before. Also, the document stipulates 15 zones that will be used by US troops, and individual objects within their borders will be available only to the United States.

Negotiations on the agreement ended in October 2023.

The agreement will be officially approved by Finland after the decision of the parliament.