Finland’s leaders announced plans to apply for NATO membership “immediately,” prompting Russia to threaten “retaliatory measures” if the Nordic country joins the Western military alliance.

On Thursday, Finnish Prime Minister Sanna Marin and President Sauli Niinistö issued a joint statement expressing their support for Finland’s membership in the alliance, which shares an 810-mile border with Russia.

“NATO membership would strengthen Finland’s security,” according to the statement. “Finland’s membership in NATO would strengthen the entire defense alliance.” The leaders also stated that Finland must apply “immediately” and expressed hope that the necessary steps to make the decision will be “taken quickly within the next few days.”

Marin’s Social Democratic Party is expected to make a decision on joining the coalition on Saturday, with the five-party coalition in Helsinki expected to make a decision on Sunday.

There is a list of minimum requirements that applicant countries must be able to meet in order to be accepted into NATO. This includes having a functioning democracy, treating minorities fairly, and being able to contribute military forces to NATO operations. Once a country meets the basic requirements, it is asked to join a Membership Action Plan, which prepares it for membership in the alliance; however, this does not guarantee a place in the alliance.

Despite NATO’s “open door policy” for prospective members, Ukraine has yet to be formally admitted. The ongoing unrest in Ukraine has alarmed NATO members, from the annexed peninsula of Crimea to the Russian-backed separatist regions in the east. During the first month of the Russian invasion, Ukrainian President Volodymyr Zelensky requested membership in the alliance. However, he admitted in March that he did not expect his country to join anytime soon. Following a phone call with Niinistö on Thursday, Zelensky praised Finland’s decision.

One of Russian President Vladimir Putin’s reasons for invading its neighbor was to prevent Ukraine from joining NATO and, ultimately, to push back on the alliance’s eastward expansion, which appears to have backfired on the Kremlin leader. Instead, Putin’s aggressive foreign policy has prompted other European countries, including Finland and Sweden, to apply for NATO membership. According to a poll conducted by the Finnish broadcaster YLE, public support for the country’s membership in NATO has increased from 20-30 percent to 76 percent since the conflict in Ukraine began on February 24.

Niinistö told a press conference on Wednesday that Moscow was solely to blame: “You caused this. “Take a look in the mirror.”

In response to the announcement, the Kremlin stated that Finland’s membership was “definitely” a threat to Russia. “As we have repeatedly stated, NATO expansion does not make the world more stable and secure,” said Kremlin spokesman Dmitry Peskov. “[Russia’s reaction] will be determined by the scope of this expansion process, as well as how far and close to our borders the military infrastructure will be moved.”

“Finland joining NATO is a radical change in the country’s foreign policy,” Russia’s foreign ministry added in a statement. Russia will be forced to take retaliatory measures, both military-technical and otherwise, in order to prevent threats to its national security from emerging.”

Finland’s Foreign Minister, Pekka Haavisto, told the European Parliament’s Foreign Affairs Committee on Thursday that the country’s decision was prompted by Russia’s unpredictable behavior and willingness to conduct “high-risk operations.”

Meanwhile, Sweden is expected to make an announcement about joining the alliance in the coming days. On Thursday, British Prime Minister Boris Johnson announced that he had signed security treaties with both Sweden and Finland. Johnson stated that the agreements would help both countries if they were threatened.

When asked if British troops would be sent to Finland in the event of “possible conflict in Russia,” Johnson replied, “I think the solemn declaration is itself clear.” And what it says is that in the event of a disaster or an attack on either of us, we will come to each other’s aid, including with military assistance.”