The Baltic Sea is more than just an inland sea—it is a historical geopolitical stage. From Viking Age seafaring to a NATO-dominated region, this brackish sea has been a lifeline for the peoples and nations of Northern Europe. It has served as a hub for economic and political relations, from the Hanseatic League’s dominance in the Middle Ages to the power struggles of the Swedish Empire, Poland-Lithuania, and Russia, as well as the dramatic shifts brought by the Napoleonic Wars, the two World Wars, and the Cold War. Yet, the Baltic Sea has also been a place of cooperation. Following the fall of the Soviet Union, the region experienced integration and growth—an evolution now once again challenged by new geopolitical tensions. How will the future be shaped by energy flows, climate change, and military strategies?