In 2018, after a century of bilateral relations, Poland and Denmark finally delineated their borders in the middle of the Baltic Sea. After the reestablishment of the Polish state in 1918, its ties to Denmark had been quite turbulent. The great powers established the conditions of the playing field. As small and intermediate European states, the two partners tried to keep their relationship as amicable as possible. Danish investors were active early in the establishment of the Port of Gdynia as well as road infrastructure during the interwar years. Though WW2 dramatically reduced the number of relations, Polish Intelligence was still active in Danish anti-Nazi resistance. During the Cold War, the so-called Polish People's Republic and the Kingdom of Denmark found themselves in opposing camps. The Polish armed forces played a decisive role in planning wartime operations against Denmark and Schleswig-Holstein. After the Cold War, the former opponents became close allies and partners in the European integration process. This book brings together prominent scholars from Denmark, Poland, and Germany to analyze diplomatic, military, intelligence, and cultural relations across the Baltic Sea from the end of WW1 to the days of NATO cooperation.