Justin Scheck and Thomas Gibbons-Neff

Billions are pouring into a clubby, secretive arms market. With Pentagon cash and unusually close Ukrainian military ties, Marc Morales has few peers.

In its hunt for weapons, Ukraine has rolled back anticorruption rules and turned to people once seen as relics of an anything-goes era.

People who would not be allowed anywhere near the battlefield in a U.S.-led war are active on the Ukrainian front, with ready access to American...