Western powers are preparing new sanctions against Russia for its aggression against Ukraine in the sea of Azov, but maybe they don’t need to bother: Russia is good enough at hurting itself.

Russia’s arrest last week of the American founder of Baring Vostok, an investment fund focused on Russia, should spook the last few investors who remained tempted by the economic and financial promise of a resource-rich country despite its permanent disregard for the rule of law.

Michael...