Finland and Sweden seem to be all set to become the newest member of defence alliance North Atlantic Treaty Organization (NATO). And Russia– their neigbour to the east– may not be happy about it.

If leaders of both Finland and Sweden agree to the accession in the coming days and make a proposal, the odds may be in their favour. Media reports suggest that it is unlikely any existing NATO country would be against the new inclusions.

Also Read: Sweden accuses Russia of violating airspace weeks ahead of NATO bid

But why is it bad news for Russia?

When NATO was formed in 1949, its core purpose was to tackle Russian aggression in the east. That continues to be true. Article 5 of the NATO charter codifies the concept of “collective self-defense.”

This would mean that an attack on any country that is part of NATO will be considered an attack against the whole alliance, which currently has 30 members. This could potentially foil Russia’s ambitions to expand towards the west.

Moreover, if Finland and Sweden join the alliance, Russia would find itself completely surrounded by NATO countries in the Baltic Sea and the Arctic. The two countries would also now be formally in a military alliance with the United States, considered to be Russia’s leading adversary.

Russia currently shares its borders with five countries that are members of NATO. The shared border expands to nearly 755 miles. If Finland joins, the shared border would increase to nearly 800 miles.

The Kremlin has warned of “military and political repercussions” if the Swedes and Finns decide to join NATO, according to reports from Associated Press.

Also Read: ‘If Finland, Sweden join NATO’: What to make of Russia’s nuclear threat?

Sweden and Finland have sought — and received — assurances of support from the United States and other NATO members in the application period should they seek membership.

Both countries feel they would be vulnerable in the interim, before they’re covered by the alliance’s one-for-all, all-for-one security guarantees.