When flames enveloped Germany’s parliament on the evening of 27 November 1933, six days before national elections, it was a political gift to Adolf Hitler, the recently appointed chancellor. The arson attack on the Reichstag by an “enemy within” secured his re-election and gave him the pretext to grab the dictatorial powers he craved. Whether that gift fell into the Nazi leader’s lap by chance or was placed there via a covert false-flag operation has been the subject of bitter historical disputes ever since. Now, 90 years later, the body of the young communist whom historians have traditionally regarded as...