Murray Leinster 
Invaders [PDF ebook] 

When we imagine establishing contact with an alien race, we usually assume that the interstellar interlopers would be light-years ahead of us in terms of technological sophistication. But what if our new alien acquaintances were astoundingly average in their appearance and intellect? That’s the premise of "The Invaders, " a tale from Murray Leinster that puts a new spin on the first contact genre of science fiction.