I can corroborate (for any lurkers and onlookers) that the modern state having a monopoly on violence is an often-accepted idea.

For example, this is professor Wael Hallaq of Columbia University describing some defining characteristics of the modern state:

"there are five form-properties possessed by the modern state without which it cannot, at this point in history, be properly conceived. These are:

(1) its constitution as a historical experience that is fairly specific and local;

(2) its sovereignty and the metaphysics to which it has given rise;

(3) its legislative monopoly and the related feature of monopoly over so-called legitimate violence;

(4) its bureaucratic machinery; and

(5) its cultural- hegemonic engagement in the social order, including its production of the national subject"