Robert O. Paxton said:
When the government of the well-meaning but overwhelmed Ivanoe Bonomi, an associate of Giolitti's center-Left, lost a vote of confidence in February 1922, it took three weeks to find a successor. Finally an even more subaltern Giolitti lieutenant, Luigi Facta, reluctantly assumed the prime ministry. His government lost its majority on July 19. When the emergency came, Facta was serving in only a caretaker capacity.
Nevertheless, the prime minister began vigorous countermeasures. With the king's approval, Facta had already reinforced the Rome garrison with five battalions of disciplined Alpine troops. Now he ordered police and railroad officials to stop the Fascist trains at five checkpoints and began preparations to impose martial law.
Meanwhile Mussolini quietly left the door open for a political deal. Several old political warhorses were trying to defuse the crisis by "transforming" Mussolini into a mere minister within yet another liberal-conservative coalition cabinet. The aged deal maker Giolitti was widely regarded as the most plausible savior (he had evicted D'Annunzio by force in 1920, and had included Mussolini in his 1921 electoral list), but he was in no hurry to reassume office, and Mussolini remained noncommittal in meetings with his representatives.
. . .
Prime Minister Facta's emergency measures nearly succeeded in blocking the Fascist march in October. Four hundred police stopped trains carrying twenty thousand Blackshirts at three of the checkpoints--Civita Vecchia, Orte, and Avezzano. About nine thousand Blackshirts who evaded the checkpoints or continued on foot formed a motley crowd at the gates of Rome on the morning of October 28, poorly armed, wearing makeshift uniforms, short of food and water, and milling about in a discouraging rain. "In ancient and modern history, there was hardly any attempt on Rome that failed so miserably in its beginning." [Martin Broszat, 1983]
At the last moment King Victor Emmanuel III balked. He decided not to sign Prime Minister Facta's martial law decree. He refused to call Mussolini's bluff and use the readily available force to exclude the Blackshirts from Rome. He rejected Salandra's last-minute efforts to form a new conservative government without Mussolini, who by now refused Salandra's offer of coalition. Instead he offered the prime ministry directly to the young upstart Fascist leader.
Mussolini arrived in Rome from Milan on the morning of October 30, not at the head of his Blackshirts, but by railway sleeping car. He called upon the king clad incongruously in morning coat and black shirt, a sartorial reflection of his ambiguous situation: partly a legal claimant to office, partly the leader of an insurrectionary band. "Sire, forgive my attire," he is said to have told the king, mendaciously, "I come from the battlefields."
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 own 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."