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Why did the king thus rescue Mussolini from a rashly overplayed hand? Mussolini had cleverly confronted the sovereign with a hard choice. Either the government must use force to disperse thousands of Blackshirts converging on Rome, with considerable risk of bloodshed and bitter internal dissension, or the king must accept Mussolini as head of government.
The most likely explanation for the king’s choice of the second option is a private warning (of which no archival trace remains) by the army commander-in-chief, Marshal Armando Diaz, or possibly another senior military officer, that the troops might fraternize with the Blackshirts if ordered to block them. According to another theory, the king feared that if he tried to use force against Mussolini, his cousin, the duke of Aosta, reputed to be sympathetic to the Fascists, might make a bid for the throne by siding with them. We will probably never know for sure. What seems certain is that Mussolini had correctly surmised that the king and the army would not make the hard choice to resist his Blackshirts by force. It was not Fascism’s force that decided the issue, but the conservatives’ unwillingness to risk their force against his. The “March on Rome” was a gigantic bluff that worked, and still works in the general public’s perceptions of Mussolini’s “seizure of power.
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Robert O. Paxton (The Anatomy of Fascism)