Geopolitical Characteristics of now Denmark, so to speak. the crossroad at the gate of the North and the Baltic Seas, has indeed thrown a long shadow on some major event in course of its history:

• For the Viking Age, the (too early collapsed) dynasty of Gudfred "the Great" (d. 810 CE) and Jelling Dynasty became the pioneer of the state formation process in Scandinavia, as I summarized in: Just how different were Viking era Scandinavian countries from each other? and The Danes or Vikings from later Denmark would often raid the Frankish Empire and later what would become the Holy Roman Empire. How were they so successful and how did the Danes avoid being conquered by the big powers?. While the latter is famous for the founder of "North Sea Empire" under Cnut (Knud) the Great (r. 1016/19-35), it is worth noting that his father Svend Forkbeard and Harald the Bluetooth had already consolidated their political hegemony beyond the boundary of the kingdom to some extent, as testified by the famous "large" rune stone of Jelling (linked to the official site of National Museum of Denamark: ""King Harald ordered these monument [kumbls] made in memory of Gorm, his father, and in memory of Thyra, his mother; that Harald who won for himself all of Denmark and Norway and made the Danes Christian."
• For High Middle Age, the rapid consolidation and sudden fall of the Baltic dominion (Empire?) of Denmark, as illustrated in: I read something referencing the "Mortgage of Denmark", where a King basically 'sold' different parts of the country to German rulers and subsequent Kings had to buy the land back. Is there somewhere I can learn more about how this happened? How does a country mortgage its territory?, illustrated both the geopolitical strength and weakness of medieval kingdom of Denmark at this crossroad the best, I hope.
• In Later Middle Ages and Early Modern Period, as I also summarized before in: Have the West Coast of Sweden always been a thing?, the history of Denmark was characterized by its rivalry with Sweden, including the control of Øresund Strait. The Sound toll register (now also compiled as online edition), recording the toll collected at Helsingør from the 15th to the 19th century, is a testimony of this "advantage" of Denmark during this period as well as one of the most important sourced for the economic history of early modern Europe.