Ulla-Korsa: Klendath ... Klendath?
Klendath: Here my lord.
Klendath: Nothing is working and we are all that survive of the bridge crew. The escape shuttle is powering up.
Ulla-Korsa: The Hell with that. Hand me my blaster. I'll take down whomever is foolish enough to board us while I live! The blaster! Now.
Klendath: Yes ... the blaster ... here!
Ulla-Korsa: ... you miserable ...
Klendath: Actually I'm feeling pretty good right now. I want you to know something oh benevolent master ... your kingdoms are a joke. We ... my people run the show. We give you the technology we decide to give you. You are a bunch of damned barbarians. You may have beaten us at war but you lost at peace. We have worked our way into all levels of your culture until we are indispensable ... till we run the show! A shutdown here a failed rive there and you lose the war against the humans. Yet you call us slaves and servants. Idiots ... we only needed you to keep the slugs and the Videni off our necks and free ourselves for true research. What do you say to that?
Ulla-Korsa: ... ...
Klendath: Well ... CRAP! Dying before I could rant. Lousy warrior scum! What ...?
<Clunk ... clunkclunk clang!>
Klendath: I don't want to know what that is. To the Klendath-pod!
Ulla-Korsa: Someone should tell the brilliant mastermind the difference between a kill and a stun setting. I will survive this if only to pay the little son of a bitch back!
Barbarians in space are my second favorite space opera trope (pirates rule, sorry). In many many tories they are put in a position to gain enough technology to invade and topple far older and more advanced cultures.
Let me explain what I mean by barbarians in space. They are people who came into the galactic community late using technology they borrowed, begged or stole from interstellar capable beings. They are new to the scene and not cosmopolitan at all, meaning they do not have much experience contacting other beings or with multispecies conventions (like signals meaning surrender or the standard airlock design). they don't have to be warlike or violent but most people think it's more fun that way.
Larry Niven's Slavers are an example of barbarians in space. They used psionic powers to take over other races and steal their technology and (even more creepy) their minds and wills. Personally they were kind of dumb, closeminded (no pun intended) and arrogant.
In a world where the Roswell incident was a real UFO crash and we were in fact back engineering the wreck's systems we would be on the verge of being barbarians in space. The best case of humans being the barbarians for my money is 'The Road Not Taken' by Harry Turtledove. It's available online and good reading so I won't spoil it here but it sums up a major point of being a successful barbarian: technology.
To be a credible threat the barbarians need an edge. The Slavers had psionics. Other races might have highly advanced technology but lack developed space transportation systems (maybe they just like things at home.) I'm thinking about the Golden Age version of the Kryptonians here. Highly advanced and superhuman specimens who had no interest in space travel because they had it so good. If another race made contact and the planet didn't explode the galaxy would be under a Kryptonian flag.
In some cases warrior spirit or raw courage gives the barbarians their edge. I'm not so sure these will out against laser rifles and battle armor when you have swords and chainmail. Poul Anderson managed to pull it off in The High Crusade, then again he was Poul Anderson! Mnay others have claimed humans are a unique mixture of technology and primitive urges and a force to be reckoned with. That depends on the rest of the galaxy. Any race that ascended to the top of the food chain must be pretty dangerous though and in a race full of dangerous aliens I doubt anyone would become so evolved they would forget their bloody bloody past. Some cultures might outlaw war. This may not be a stable state of affairs. Take out enough of their ships and conquesr enough of their worlds and you might see what high tech really means.
Sometimes the edge is not a strength the barabarians have but a weakness the 'civilized' galaxy has. GDW's Imperium had just grown too big to pay attention to those upstart Terrans until it was far too late (we also bred like rats.) Being preoccupied cost Great Britain the War of 1812 (or at least let the Americans survive it.)
Finally technology might not be everything. Perhaps energy weapons seem superior to slug throwers but just are not worth the extra cost, training time, and maintenance. So the troops with slug throwers will win the war even if they loses some initial battles.
The most dangerous thing that any so-called barbarians can do is not attack or flank his enemy or torture the prisoners of a hundred worlds though. The most dangerous thing they can do is learn. That might be all the advantage they need.