Sentinel Force I by Robert Price

One man's ambition for ultimate power has torn the universe a part. The warlord can subdue every planet except for earth and their protectorates worlds, because their defensive systems are too strong. It may be a matter of time before he can starve them out, they may hold out for years. They cannot buy badly needed supplies, because his forces have them surrounded.

Now war spills into the past, the warlords thinking, if he cannot conquer the Earth in his time then he shall go into the past. To kill the inventor of the defense system, this would change time and events, the system would never be completed and the warlord could take what he wanted.

Scientist learn of his plan, defy death to steal the most advanced ships of that period to send them back to a reluctant hero. This begins the building of a force called the Sentinels; they are humankind's last hope for survival.