Inside the stunning growth of Russia’s Wagner Group

American and European allies are mobilizing to thwart the rapid expansion of the Russian paramilitary group known as Wagner, run by a Putin-affiliated oligarch, as it captures key cities for Moscow in Ukraine and spreads its influence to Africa and other corners of the world.

With tens of thousands of fighters, many of them now battlefield-trained, the Wagner Group’s emergence as a rogue military threat could become a serious global challenge in years to come, U.S. and European officials said.

Wagner’s stepped-up activities in Africa, where its trained fighters perform...