If I had checked for the decrease in amount of available persian oil, I would have understood that Darius upgraded his cavalry into tanks

I was wrong in my assumption that Persia rush built the research lab.
But they got Combustion not merely because of tanks and destroyers, it seems they entered Modern Era through Atomic Theory. Askia believes that one of persian cities is already working on Manhattan Project.
Catherine, Askia advises you to stay neutral in the erupted conflict between Persia and Songhai. Songhai can be a good defensive buffer between Persia and Russia. We can fight back persian tanks by our anti tank units, and persian bombers by our fighters. We can delete persian advance, and make them suffer considerable casualties. However, when nuclear weapon of Persia comes into stage, Songhai will be defeated.
Meanwhile Russia has some time to tech up, seize city-states on the east and train more advanced units.
If Catherine helps Persia to defeat Songhai, then she will soon be forced to face nearly alone a full-power persian army buffed with persian GA bonus and supported by nuclear weaponry.
PS Playing as Songhai, I was trapped between the Scylla (Tabarnak) and Charybdis (EEE_BOY)