Short answers, pulled from the story.
Scandinavism is an ideology supporting various degrees of cooperation among the Scandinavian countries. It encompassed literary, linguistic, and cultural efforts to promote a shared Scandinavian past, a common mythology, and the dialect continuum descending from Old Norse, as well as a broader political project for Scandinavian unity that was most active in the mid-19th century.
Pan-Scandinavianism as a modern political movement originated in the 19th century, though cultural precursors had been spreading in literary and scientific circles roughly a century earlier. Danish and Swedish university students initiated the organised movement in the 1840s, with a base in Scania.
Scandinavism collapsed in 1864 when the Second Schleswig-Holstein War broke out and King Charles XV of Sweden, despite championing pan-Scandinavianism, failed to send military support to Denmark. That failure effectively ended Scandinavism as a state-building project.
Hans Christian Andersen wrote Jeg er en Skandinav, meaning "I am a Scandinavian", drafting the text in July 1839 on the Danish island of Funen. Composer Otto Lindblad set it to music, and the piece was published in January 1840; its popularity peaked in 1845.
The Clara Lachmann Foundation was established in 1923 with the goal of promoting Scandinavian unity through culture. It represents the survival of the cultural strand of Scandinavism long after the political project collapsed following the 1864 war.
Scandinavism paralleled the 19th-century unification movements of Germany and Italy in ambition, but unlike those movements the Scandinavian state-building project was not successful and is no longer pursued.