What did Russia promise Syria after the Il-20 loss?

What did Moscow decide to do after losing a military aircraft over Syria? On 24 September 2018, Russian officials stated that Russia would send the S-300 air-defence missile system to Syria within two weeks, and that it would begin jamming radar and satellite signals of planes attacking Syrian territory from the Mediterranean.

The trigger

The announcement followed the events of 17 September 2018, when the Israeli Air Force struck multiple targets in government-controlled western Syria. Syrian air defences responded to the Israeli jets and accidentally shot down a Russian Il-20 military aircraft, killing all 15 crew aboard. Russia blamed Israel, accusing its air force of hiding behind the Russian aircraft and so putting it in the line of fire. The Israel Defense Forces acknowledged striking a Syrian weapons facility and expressed sorrow for the loss of life caused by Syrian fire.

What the measures signalled

The two steps were different in kind. Delivering the S-300 was a commitment to supply hardware, with a stated timeline of two weeks. The jamming of radar and satellite signals was aimed at aircraft operating from the Mediterranean against Syrian territory. Both were presented as a response to the downing of the Russian plane.

The strikes of 17 September had come a few hours after a Russo-Turkish agreement to create a demilitarised zone around Idlib Governorate, which postponed an imminent offensive by Syrian forces and their allies. The announcement of 24 September thus came in a theatre where Russian diplomacy and military risk were unfolding at the same time.