What happened in Beirut?

Can someone explain to me what happened in Beirut? This is mentioned when Captain Price and Kamarov meet each other in Call of Duty: Modern Warfare 2019.

Not to mention, in Call of Duty 4: Modern Warfare on the second mission it is also mentioned.

So what is Beirut?

We don't know.

This has never been explained in any official Call of Duty media. It is implied that there was a mission in the past in Beirut (obviously) in which Price helped Kamarov out, thus the whole "You owe me" sort of deal.

In 1983 the US marines barracks were attacked by a suicide bomb in Beirut killing more than 200 US marine.

You may read about what happened that day on Wikipedia.

https://en.m.wikipedia.org/wiki/1983_Beirut_barracks_bombings