"Violence, naked force, has settled more issues in history than has any other factor, and the contrary opinion is wishful thinking at its worst." Do historians agree or disagree with this Robert Heinlein passage from Starship Troopers?

It's pretty hard to say for certain without narrowly defining terms, but we can take a look at the man who wrote the line for some idea of the view he was trying to propagate.

First of all, let's look at the full quote, and character speaking it. It is spoken, during a lecture to his class, by Lt. Col. Jean V. Dubois (Ret.). Dubois is often held up as a cipher for Heinlein himself within the context of the book, and the full quote lines up fairly well with some of Heinlein's core beliefs. Dubois goes on: "Breeds that forget this basic truth have always paid for it with their lives and their freedoms. [...] A soldier accepts personal responsibility for the safety of the body politic of which he is a member, defending it, if need be, with his life. The civilian does not."

Heinlein was a libertarian with a militaristic streak (he graduated from Navy, and was a sailor in the 30's), and Starship Troopers, as the title suggests, is a chance for him to explore the militaristic aspects of his philosophy. Heinlein wanted to show in the novel, and through the words of Dubois especially, not so much that the lesson of history is that violence is always effective, but that those who bury their heads in the sand and do not properly acknowledge the critical role of force in the shaping of human affairs, are doomed to be destroyed and enslaved. He is warning the reader that ignoring political reality for the sake of pacifist fantasy will, in his opinion, lead to disastrous consequences.

In the novel, Heinlein therefore creates a sort of "Plato's Republic" of his own, based on this key admonishment. The world of Starship Troopers see two key classes of people: citizens and civilians (or soldiers and civilians). Those who do not wish to engage in force and violence, who don't have the stomach for it, have the same rights as those who do -- but the ones who are willing to fight to defend the country are given a higher status, with the privilege to actually vote and have a say in the mechanism of government (which is, after all, simply an organization of force on behalf of society). The novel was inspired by, and written in the wake of, calls for unilateral cessation of nuclear testing by interest groups and anti-war groups in the United States. Heinlein wanted to offer a countermanding opinion.

So, Dubois' real purpose with the speech not so much for a historical survey, but for teaching the viewpoint to his students (unsuccessfully, as he later admits) that "those who beat their swords into ploughshares usually end up ploughing for those who kept their swords." Whether or not that fact has been borne out by historical reality is, again, difficult to say.