y evolution of the Guard under the Julio-Claudian emperors, up to the death of Nero.
Every state that possesses an army finds itself confronted with a fundamental political challenge: the risk of a military coup. The risk is inherent in the nature of an army. Reduced to its essential terms, an army is a body of men (invariably men, save in rare emergencies and very recent years) who are equipped, trained, and organized to fight. An army is therefore at great advantage -- usually overwhelming advantage -- in any armed confrontation with a group that lacks these characteristics, or has them in markedly lesser degree. Put simply, an army can defeat and either kill or impose its will on a political leadership, a general population, or any faction of either that is not itself organized as an army.
States have found, or attempted to find, various means to deal with this latent threat from their own armed defenders. A few, facing no substantial foreign threats (or none they had any serious hope of fighting off in any case) have done without armies. These states are few -- even fewer in practice than in name, since nominally army-less states frequently have some sort of national paramilitary police who have nearly as much advantage over civilians as a "real" army would have.
A few other states, who by geography and strategic requirements have primarily maritime defenses, have been able to rely on a navy as their primary force. Navies, historically, seem far less predisposed to overthrow their governments than do armies; in the words of William H. McNeill, "sailors fresh from long weeks at sea have other things in mind than seizing power on behalf of their commanders" (p. 261). For the same reason (and because navies are not primarily equipped to fight on land), a loyalist navy might seem little protection against a rebellious army even if the latter were much smaller, but in practice the protection they offer has been subs...