- The First Tyrannic War (745 M41): The Imperium makes first contact with the Tyranid race when Hive Fleet Behemoth invades the Eastern Fringe and annihilates planet Tyran; Behemoth is defeated, though at great cost, by the Ultramarines at the Battle for Macragge.
- The Second Tyrannic War (990-993 M41): Hive Fleet Kraken invades and is, again, narrowly defeated.
- The Third Tyrannic War (997-ongoing): Hive Fleet Leviathan, the largest Tyranid force yet, invades.
Collectively, the Tyrannic Wars have seen a massive mobilization of Imperial forces and is among the highest prioritized conflicts in the Imperium. Deployed primarily to the Eastern Fringe, casualties among Imperial Guard regiments have been so high that it seems as if the Tyranids are annihilating them faster than they can be deployed.