King Ghidorah's Campaign was an attempt by King Ghidorah to destroy and terraform the Earth into a world of his own liking.
This campaign actually started thousands of years ago when he first arrived on Earth. To do this, he attempted to enslave the Titans into doing his bidding and defeat his arch-nemesis Godzilla who species were considered the be the alpha species had rivaled him to thwart his threat.
The campaign ended in 2019 when Godzilla had killed Ghidorah during the Battle of Boston, ending his reign and the threat he posed.
After King Ghidorah's demise, Godzilla had reclaimed his rightful position as King of the Monsters. However, the consequences of King Ghidorah's reign of terror was evident. The Titans had attacked several of the world's major cities and thousands died. The fight between King Ghidorah and Godzilla had reduced most of Boston to rubble.
However, despite this catastrophe, Godzilla had maintained control over the Titans, who were using their abilities to heal Earth's ecosystems and environments and maintain balance.