If Sweden and Finland join NATO, Russia could deploy nuclear weapons to the Baltic in particular, ships with appropriate missiles.
Source: The Deputy Chairman of the Security Council of Russia Dmitry Medvedev
“If Sweden and Finland join NATO, the length of the alliance’s land borders with Russia will more than double. Of course, these borders will have to be strengthened. We will strengthen the grouping of ground forces and air defense, and deploy significant naval forces in the waters of the Gulf of Finland. In this case, there will be no question of any nuclear-free status of the Baltic – the balance must be restored,” Medvedev wrote.
See also: Russia deploys troops on the border with Finland following its declarations of joining NATO