Russian President Vladimir Putin and Chinese leader Xi Jinping oversaw an ambitious joint military exercise in China this summer, which along with reported collaborations in aviation, undersea and hypersonic-weapons technology point to a solidifying defense alignment, according to military analysts.

U.S. officials and military specialists say it is difficult to pin down the level of collaboration between two nations that tightly control information, and whose actions are increasingly opaque to outsiders. But Western officials and defense experts are growing more convinced of the closer relationship based on recent economic alliances, military exercises and joint defense development, as well as the few public statements from government leaders.