ST. PETERSBURG, June 6 -- The claim arrived at the most visible moment available: a plenary stage at the St. Petersburg International Economic Forum, before an audience of heads of state, energy ministers, and the kind of business delegation that takes notes. Vladimir Putin said on Friday that a significant portion of the Indian army is equipped with Russian-made weapons and equipment, framing six decades of Soviet and post-Soviet arms supply as a living tradition that continues to define the bilateral relationship. "This has been the tradition since Soviet times," he told the gathering.

That tradition, by the most credible measure available, is eroding faster than Moscow appears willing to acknowledge in public.

According to the Stockh...