Let's see what type of compulsions
1)Due to Turkey's involment.
2)Due to other islamic countries involment.
3)Due to involment of Syrian fighters
4)Due to NATO
5)Due to the insults face by russia for its attempts to cease fire.
6)Due to the treaty with Armenia.
7)Due the long standing russian influence in this region.
All the above mentioned factors made it compulsory for russia to get involve in this war.Russia initially tried it to avoid entry in this war.Tried various ways so that both the countries should follow cease fire.But twice both the countries broke all the agreements they made for cease fire.It was very very depressive for the russian attempts made for peace in this region.Both the countries blamed each other for cease fire violation.
Due to the help from Turkey,other islamic countries and Syrian fighters,the situation got even worsed.In the past russia had worked very hard to keep the surrounding region out of reach of terrorists influence.But now due to Syrian fighters and Turkey's involvement this region can go under the influence of terrorists activities.So it will be a very alarming situation for russia in near future.To keep all these things in control russia tried for cease fire agreement also.But all the russian attempts wasted.So finally russian entry become very very important in this war.