Conquest of the Nine Realms
The Conquest of the Nine Realms was a series of wars during which Asgard invaded and subdued the other realms, leading to the creation of the Asgardian Empire.
Although it is referred to as the conquest of the Nine Realms, not all the Nine Realms were conquered - Midgard and Niffleheim were never invaded, although the Asgardians have traditionally considered Midgard their propety despite this.
The conquest of the Nine Realms was begun during the reign of Bor, but the majority of the wars were lead by Odin or Hela.
Conflicts
The conflicts generally accepted to constitute the Conquest of the Nine Realms are:
Aesir-Vanir War
First Dark Elf War
Alfheim Campaign
Surtur War
Second Giant War
Asgard-Jotunheim War