The Supreme Leader, Ayatollah Ruhollah Khomeini, purged the ranks of the Shah’s old military, as well as many of the groups that had helped bring the Shah down, but later turned on him, when they began to oppose his rule. The Supreme Leader restored order with the formation of the Iranian Revolutionary Guard Corps, an elite military unit which he created and kept under his control. However, though loyal to the Supreme Leader, the Guards do not always follow orders in matters of internal strife, such as orders to crack down on dissenters in street protests, or demonstrating in public places such as universities.
A violent entity created in 1979, who Khomeini named the Basij Mostazafan, or “mobilisation of the oppressed”. The Basij militia became useful to the Iranian leadership, trained by the IRCG and indoctrinated with the Supreme Leaders Shia ideology, they are willing to sacrifice their lives for both leadership and state, and have no qualms about going against their fellow countrymen.
The Basij drive motorbikes into anti-regime protests and beat the participants with clubs and chains. If necessary, they open fire on peaceful demonstrations, something the Guards balked at, as its commanders preferred to stay a popular force amongst the people.
In August 1994, the Guards were ordered to open fire on protestors in Qazvin, who were demonstrating against living conditions, during Iran’s worsening economic crisis. The Guards refused to leave their barracks, not wanting to be involved in cutting down unarmed protestors with automatic weapons. The Basij was called in to confront the crowds.
Their devotion to Khamenei is fanatical and they show no mercy towards those the regime class as dissidents, and in the case of the Qazvin protest, Basij units killed an estimated 40 people, and injured at least 400.
The need for the Basij first became apparent to Khomeini at the end of the revolution, when he needed a militia that would be totally loyal to him to back up the IRGC, after he had purged the Iranian armed forces, feeling that many were still loyal to Shah Pahlavi, and that they might conspire to overthrow him.
This resulted in a severely weakened military, which turned out to be a catastrophic mistake in those times of constant upheaval, which in a short while would lead up to the Iran/Iraq War.