What two specific historical incidents dramatically increased the hazard posed by orbital debris?

A 2007 Chinese missile test and a subsequent Russian-American satellite collision

The issue of space pollution was significantly exacerbated by specific, high-impact events that created massive clouds of dangerous fragments. The first major contributing event mentioned was a missile test conducted by the Chinese government in 2007, which resulted in the shattering of an old weather satellite into roughly 150,000 pieces of debris. This was compounded two years later by a collision between operational Russian and American satellites. These two events dramatically increased the population density of the orbital graveyard, contributing to the approximately six thousand tons of material estimated to currently occupy Low Earth Orbit and increasing the required mitigation efforts for current and future missions.