A new documentary by the BBC (below) will end any remaining doubt about the presence and activity of Iranian forces there.
In this documentary, you not only see Iranian advisers from the IRGC taking part in some of the fighting, you also see them admitting that pro-Assad militia are being taken to Iran for training.
Iran is one foreign force in this war. There are other foreign forces too. The Saudis and the Persian Gulf Cooperation Council (PGCC) countries have also been helping the Sunni militias.
However until now, the Iranian regime had been very careful not to be seen taking part in the killing of other Muslims in the Middle East. In fact over it has bent over backwards by trying to portray itself as the defender of all Muslims, including Sunnis. This is why it was so happy when it had the support of Hamas.
With its support for Assad and his indiscriminate killing of Sunnis (even using Chemical weapons), those days seem to be over. In the Sunni majority Middle East, the Iranian regime will now be seen as another oppressor of Muslims, despite its religious facade.
You can watch the documentary here