The Star Conquerors, Ben Bova, John C. Winston Company, 1959
This science fiction novel is about an interstellar war between the Terran Confederation and the much larger star empire of the Masters.
Things are not going well for the Terrans. The Saurians (who do the actual fighting under the control of the masters) outnumber the Terran forces by several times. The Terran fleet is in bad shape. Geoffrey Knowland, whose father, Heath, runs the whole Earth defense effort, is authorized to take what's left of the fleet and go on the offensive. They begin to have an effect, destroying a base here, and an outpost there.
A million years ago, mankind had spread out among the stars, with colonies on a number of planets. They ran into a highly advanced race, the Others, who wiped out mankind's star empire, and caused an ice age on Earth. These surviving colonists evolved into separate humanoid societies. A resident of one of these societies, Alan Brakerman (his actual alien name is nearly unpronounceable), escapes from a Masters-ruled planet, and joins the Terran forces.
The Terran forces liberate a number of societies from the Masters, but the natives aren't exactly jumping for joy. Life was peaceful under the Masters, but it was also quiet. Scientific exploration was subtly suppressed, and even the birth rate was stagnant. The Komani, one of those societies, is totally based on war. They have a very hard time waiting for permission from Knowland to fight.
After causing major problems for the Saurians, and the Masters, the Terrans get a message from the Masters offering to sue for peace, each side can keep the territory it has. The Terrans decline the offer; the Masters may be bloodied, but they will regroup and wipe out the terran Confederation. The decision is made to take the fight to the home world of the Masters. When they meet an actual Master, the Terrans get more than they expected.
Sometimes, a person just has to read some old-fashioned space opera, full of interstellar space battles, where the outnumbered Earth forces bravely do battle against the implacable foe. This one is quite good.