Wars and various burning of bondholders (mostly by the government buying up debt at market price and cancelling it) dominate until the post-war era. Post-war I don't know. The decrease in debt/gdp around 1945 and 1960 appears rather sharp but perhaps it is just increasing GDP, can't find anything about bonds written down. The last debt peak around is at least the bank crisis of the early 90ies with austerity and introduction of mass unemployment. Then follows a decrease in debt/GDP as a result of a mercantilistic policy of wage depression and a debt reduction target.

Sweden's finest (and perhaps only) collaborative, leftist e-newspaper Synapze.se