After being demilitarized in 2005, Sweden re-introduced permanent troops to Gotland in 2016, following Russia’s annexation of Ukraine’s Crimean Peninsula in 2014. Sweden also became NATO’s newest member in March — meaning it is covered by the alliance’s Article 5 guarantee that all other members come to each other’s defense if they are attacked.
Here’s his justification, so he doesn’t have to make one up out of thin air:
So, you see, it was Russian territory for almost a month.
https://en.wikipedia.org/wiki/Gotland#Early_modern_period
Its position in the middle of the Baltic Sea and more or less off the coast of Kaliningrad would make it an excellent military outpost from which to control shipping and spy on their neighbors.