It's funny, really. Everyone across the galaxy knows that when the humans pick a side of a war, the other side might as well give up. No one has ever defeated them, after all. The stories are varied - humanity comes from a deathworld, and actually tamed it, unlike most deathworld-born sophonts. Everyone's seen the ships they travel in - massive, armored things, starfaring fortresses that only human allies are permitted aboard. The stories all paint lurid pictures of unstoppable warriors flying invincible war machines. It has to be true, the last species that knowingly went to war with them were the Voont, and when was the last time you saw a Voont around?
It's true that you don't see the Voont around anymore, but not for the reasons everyone thinks; humanity, when they pick a side in a war, don't go in to fight. They learned their lessons about that thousands of years ago, when they all still lived on Earth, and nearly turned it into a dead and poisoned world.
No, the survivors of that mess learned from it. They don't fight anymore; every life is precious to them. No one ever gets into a territorial dispute with humans because they pick the most hellish worlds to claim, and they only do it after making sure no one else wants it, including any proto-sophonts living there. And they learned that most species - even the belligerent ones - don't go to war for the sake of it; they do it out of need.
When humans go to war, their massive armored ships show up at every planet you control, and the next thing you know you have offers to help fix things coming in. The climate of your worlds not right? They can help. Medical trouble? They're good with a vast range of biologies. Resource woes? They'll go corral entire massive asteroids laden with whatever you need and park them in orbit for you - or dismantle them and ship them down, if you need it. And they're just so nice about it - never trying to intrude, not trying to be pushy, just there to lend a hand with whatever you need, if you happen to want a hand.
And eventually, because they're evolved from persistence predators, they win you over. You find yourself invited up to their ships so they can show you their culture, and so you can teach them about yourself. And that's where you learn that 'human' stopped being a species designation a long, long time ago. Humans are those persistence predators who learned a hard lesson, yes, but they're also all the uplifted species that adopted themselves into the human race, and the constructs that humans made, realized had developed awareness, and welcomed into the family, and all the others that humans have gone to war with over the millennia.
You won't see a Voont anymore, no. But you'll see humans whose ancestors called themselves Voont, and had the nicest collective species in the galaxy turn up and offer to help with everything, and ended up being adopted into the sprawling label of what it means to be human.
Humans don't go to war to fight anyone. Humans go to war to help people. And they've gotten very, very good at it.