The Skylark Trilogy

Skylark is a science fiction/space opera series by American writer E. E. Smith. In the first book, The Skylark of Space (first published in Amazing Stories in 1928), a scientist discovers/accidentally invents a space-drive, builds a starship, and flies off with three companions to encounter alien civilizations and fight a larger-than-life villain.
The Skylark of Space was the first of a series which continued through three subsequent books-Skylark Three and Skylark of Valeron written during the 1930s. R. D. Mullen declared that "The great success of the stories was surely due first of all to the skill with which Smith mixed elements of the spy thriller and the western story (our hero is the fastest gun in space, our villain the second fastest) with those of the traditional cosmic voyage."
The entire series describes the conflicts between protagonists Seaton and Crane, and antagonist DuQuesne, which often break into open warfare. It also includes depictions of progressively increasing scales of conflict (equalled by progressively-increased technology) between themselves, individually and collectively, and a series of non-humans bent on universal conquest. When forced to cooperate against an alien species which had conquered one galaxy and was expanding into others, the characters conclude that the universe is large enough to allow peace.