Since the president of Turkey "revealed" that his army was preparing to invade northern Syria in order to push the armed Kurdish militias from operating on the southern border of his country, an action that Iran is strongly opposed to.
Such a move by Turkey undermines Iran's exclusive authority on Syrian soil and weakens its rule over Syria.
Iran, which tried diplomacy side, with Russian help, to stop Turkey from invading northern Syria, soon realized that the Turkish leader is not taking them into account and that Erdogan was determined to carry out his plan.
For over a year now, Iran has been preparing, with the help of the Shia militias loyal to it the advancement of forces towards the conflict zone with the intention of helping the Syrian army to thwart Turkish military activity.