ya you are right, if you remember mid air collision over delhi in 1996 of a saudi 747 and a russian aircraft, at that time tcas was not compulsory to be installed on the aircraft, and it was the atc who was guiding the both aircrafts with a minimum separation height, though it was the fault of the russian crew, they didn't knew english properly and couldn't understand what the controller at delhi was saying.

After this incident TCAS was made mandatory on every aircraft....