ISTANBUL — The Lebanese Shiite militia Hezbollah and the Syrian government have enjoyed a close, fruitful relationship for nearly 40 years. But six years into the Syrian civil war, there are signs that battle fatigue and diverging strategic visions are fraying their alliance. Syrian President Bashar Assad is coming under increasing pressure from pro-Russian factions in his ruling circle to dump pro-Iranian Hezbollah, as a U.S.-Russia accord to establish a de-escalation zone in southern Syria gets underway this week. It’s a different kind of proxy war playing out in Syria: Instead of Sunni versus Shiite, or the U.S. versus Russia, it’s Russia versus Iran. “There is a pro-Moscow faction that wants Syria to be secular and includes officers who trained in Russia,” said Ayman Abdel Nour, publisher of the largest anti-Assad Syrian news portal and leader of the country’s exiled Christian community based in Dubai, United Arab Emirates. “Those who support Iran are people bought by the Iranians or [who] reached their positions of power with Iranian help.” Incidents of rivalry and strain between Hezbollah forces and Syrian government allies have been increasing since June 2016, when they openly clashed during what was supposed to be a joint operation in the Aleppo countryside. “The Hezbollah message has been: ‘Don’t think you can make a deal without us. We are on the ground, and we control what’s going on,’” said Ahmad Hardan, a 20-year-old ambulance driver from Aleppo detained by Lebanese Shiite fighters as he and his family fled their home. “They took all the young men from the cars and drove us to the opposite side of the road. All those who tried to resist were killed,” Mr. Hardan said. “But suddenly there were Russian fighter jets in the sky, the Hezbollah troops started shooting in the air, and then they let the evacuation proceed.” Russia pressures Bashar Assad to cut Syria’s Hezbollah alliance How unfortunate.