Cooperation between Iran and Russia has been growing since Russian forces intervened in Syria to help rescue Iran's top regional ally, Syrian President Bashar al-Assad in 2015. Military ties between the two countries have now expanded to Ukraine, where Russian forces are using drones supplied by Tehran. The deepening alliance is disrupting regional balances and causing worry among Gulf monarchies and Israel. 

For more on Iran's military policies you can read excerpts from this recent report published by the International Institute of Strategic Studies.