BERLIN — A century after the start of World War I — the “just war” that was supposed to end all wars — the debate about its origins has become more textured and contentious, as much in once-imperial Berlin as in once-imperial Britain, which were adversaries in 1914 and are now confronting some of the same riddles of history.

At the core of the discussion in both countries is the question: Should Germany alone bear the blame? And much as there are those who cleave to the long-held view of many that German expansionism propelled the conflict, more recent scholarship suggests that its roots were far more tangled.

“Historians no longer look simply to Berlin to explain the causes of the Great War, but increasingly to Berlin and Vienna, to St. Petersburg and London,” a group of German experts wrote recently in the newspaper Die Welt. As one of the authors, Sönke Neitzel, said in an interview, “You can’t break it down into the bad guy is this one or that one.”

In a world as multipolar today as Europe in the early 20th century, the implications go far beyond academic debate....