Arendt's long essay/short book "On Violence" notes that war has become unglamorous and ineffective as a political force, yet it remains because we have not found an adequate replacement for this. This is perhaps understood as a more politically-minded equivalent of William James's idea 60 years earlier that we need to find a "moral equivalent of war" that will harness the cooperation and personal altruism that war can elicit, but without the horrific consequences that far outweigh the benefits.
Among the many useful concepts in Arendt's book are the definitions of power, violence, strength, force, and authority as distinct entities, despite our tendency to conflate them, or use them as synonyms. Most important is the difference between power and violence, which Arendt suggests are often found together but are in fact opposite in many ways. Specifically, while violence can undo power, it cannot build it. Violence is not simply power expressed in its most brutish fashion.
Also important is the final third of the book, in which Arendt takes apart the notion that political violence is somehow "natural" or part of the human condition.
In the end, it is this idea that is at the center of the book: violence is routinely accepted as inevitable--as a given in human society. Arendt asks us to acknowledge the much more troubling truth: violence is conscious human action. It should not be natrualized or taken for granted or romanticized, but carefully examined.
The book is well-written, yet dense and often casually drops historical and philosophical references without much explanation for the uninitiated reader. Despite that, it is readable despite its often abstract nature. It doesn't leave you with a clear call to specific action, but by openly questioning longstanding myths about violence and its alleged utility in solving political problems, it does a great service.