The Sentries were a race of sentient mechanical beings created long ago by a humanoid race in order to fight the Null, which that humanoid species had accidentally allowed to cross from a subspace region into normal space. These humanoids gave the Sentries a directive to fight the Null which they could never stop doing. This mission led them to create the Sentry Coalition as a political entity.
The Sentries would fight their war against the Null for centuries, never able to defeat it entirely but always beating it back into its subspace realm. Then, in 2381, they encountered the USS Titan, which was able to help them close the subspace breaches that the Null entered regular space through. This ended the threat from the Null, giving the Sentries control over their own destinies for the first time. (TTN novel: Synthesis)