Back in 2016, Tehran and Moscow concluded an agreement to secure military supplies and equipment from Russia. According to the agreement, Iran can purchase Russian military fighters, air and naval defense systems, and other high-tech defensive hardware. Iran also secured advanced Russian-made anti-tank and anti-aircraft systems.
In October 2018, Iran and Russia signed a five-year defense and security agreement, including a clause on increasing military cooperation, which allowed Iran to purchase new Russian weaponry. Iran has purchased and received the powerful S-300 missile system, fighter jets, and other high-tech defense systems from Moscow.
To facilitate the supply of weapons, Iran and Russia have established a direct international route from Moscow to Iran, allowing Russia to deliver the military supplies directly to Tehran without the need to have the weapons shipped via third countries.
Western powers have very little leverage over the Iran-Russia military trade route as it runs through international waters and airspace, making it difficult for other countries to disrupt the shipments. In addition, Russia’s strong diplomatic support for Iran further reduces the possibility of any outside interference. As such, there is very little the West can do to stop the shipments of arms from Russia to Iran.