Why did the Danes destroy their own ships?

The Danes destroyed their own ships so as to stop the Germans from taking them for their “own use.” Annemarie is reminded of this incident in chapter 4 of the book as she is playing with Ellen and Kirsti. Ellen wants to pretend that their dolls are going to the Tivoli Gardens in Copenhagen. Annemarie remembers how her parents used to take her to the Tivoli when she was younger and how she liked the fireworks at the gardens. Kirsti, hearing about the fireworks, is reminded of the great explosions during one of her birthdays—explosions that her mother said were “fireworks for her birthday” in an effort to protect her from the scary reality of the war. Annemarie clearly remembers the explosions that happened during Kirsti’s birthday. She remembers reading from the evening paper a day later about the source of the explosions: how the Danes had blown up their own ships so as to prevent the Germans from using them in their war campaigns against other nations.