One of my friends told me about this book/book series. Aliens aren't allowed to enter our solar system because we are too dangerous, but one ship comes to earth asking for help against one of their enemies. Humans have been used in wars in the galaxy or universe before and we were too destructive. We were labeled as weapons of mass destruction and not to be contacted or interacted with. Can anyone help me with this?
There are quite a few stories where humans are much better warriors compared to other species and used as mercenaries. A little bit more details would help in narrowing down the list.
Two examples here.
Legacy of Aledanta series by John Ringo
One group of aliens comes to earth in search of warriors to fight against another set.
The short story "With Friends Like These" by Alan Dean Foster.
Several alien races come in search of humanity and Earth after the planet and its inhabitants had been locked away for for millenia for being unable to play nice with the rest of the universe. Now they were desperately needed to defeat aggressive aliens known as Yops.
This sounds similar to the book The Excalibur Alternative by David Weber. It is available in several of the Baen Free CDs, notably the 'Mission of Honor' CD.
A single alien ship arrives at Earth, and seizes several ships worth of medieval English soldiers (knights, infantry, and archers) from a storm which would have certainly destroyed the entire fleet. These soldiers are used by the aliens to serve as a 'brute squad' of sorts. They fight battles for the aliens so that the aliens can secure mining rights (or rights to other planetary resources) on planets which, by law, the aliens themselves cannot attack.
The aliens were inspired by a different group of aliens (a different species, or possibly just a different corporation) which had previously had great success doing the same thing with a Roman legion. Their success was so great, in fact, that a law had been passed forbidding the use of humans in this manner.
The aliens who abducted the English are in violation of this law, and are simply trying to turn as much profit as possible before they are discovered.
The humans eventually discover this, as well as a secret which gives them a chance to turn the tables on the aliens.
This is the book, The Human Chronicles.
Humans may not be the strongest, or the fastest or the most coordinated, but no race in the galaxy combines all these traits into one being to such a degree as we do.
Sounds like The Damned trilogy by Alan Dean Foster.