Russia built a self-proclaimed fortress around its economy in the run-up to war—but there was a crack. Moscow depended on foreign middlemen to ferry its most strategic and lucrative export around the world: oil. Now the most-important middleman, Trafigura Group, is joining several competitors in cutting off Russian giant Rosneft Oil Co. from global oil markets . In a high-stakes move that goes farther than official Western sanctions, the Swiss commodities trader plans to stop exporting Rosneft’s crude altogether. It will cut its business with the state producer to a sliver of prewar levels, supplying only some refined products such as diesel into Europe, according to a spokeswoman.