Finland is set to become the 31st member of NATO on Tuesday, strengthening both the country and military alliance, the head of the alliance announced Monday. 

“This is a historic week,” Secretary-General Jens Stoltenberg told reporters in Brussels. “From tomorrow, Finland will be a full member of the alliance.” 

“We will raise the Finnish flag for the first time here at NATO headquarters. It will be a good day for Finland’s security, for Nordic security and for NATO as a whole,” he said, adding that Sweden will also be safer as a result. 

The historic news also drew a stern warning from Russia, who threatened to bolster its defenses near their joint border if NATO deploys any troops in its new member country. 

“We will strengthen our military potential in the west and in the northwest,” Russia’s Deputy Foreign Minister Alexander Grushko said according to state news agency RIA Novosti. “In case of deployment of forces of other NATO members on the territory of Finland, we will take additional steps to ensure Russia’s military security.”

Stoltenberg said Turkey, the last country to have ratified Finland’s membership, will hand its official texts to US Secretary of State Antony Blinken on Tuesday as NATO foreign ministers gather in Brussels. Finland will then be invited to do the same, Stoltenberg said.

A flag-raising ceremony to add the Finnish flag to those of the other members will take place at NATO headquarters on Tuesday afternoon.

Finnish President Sauli Niinistö and Defense Minister Antti Kaikkonen will attend the flag-raising ceremony alongside Foreign Minister Pekka Haavisto. 

“It’s a historic moment for us. For Finland, the most important objectiveja at the meeting with be the emphasize NATO’s support to Ukraine as Russia continues its illegal aggression,” Haavisto said in a statement. “We seek to promote stability and security throughout the Euro-Atlantic region.”

Finland’s welcome to NATO comes after voters denied left-wing Prime Minister Sanna Marin another term after losing to the right-wing National Coalition Party on Sunday.