It would appear that the natural frontier of Russia runs from Dantzic or perhaps Stettin to Trieste.

Summary

This quote suggests that the natural boundary or limit of Russian territory reaches from the cities of Dantzic (now Gdańsk) or Stettin (now Szczecin) in the north, to Trieste in the south. It implies that these cities mark the farthest extent of Russia's potential geographical expansion. The statement implies a geopolitical perspective on Russia's natural borders, potentially implying a desire for Russia to exert influence or control over the areas mentioned.