chef Rome déchassé de son règne pour peur des croix ployer l’enseigne conflit règne abandonnera le plus grand chef faillira au besoin

chief, represents a leader, political or military. synecdoche [the part is taken for the whole] for Italy driven away from the royal palace, in the present case the monarchy. by fear of crosses. A group whose distinctive sign is a cross. (The nazis) to pack one's luggage> chief. the head of a group, an army or a country the head chief. In the present case, the head of a country after the king. conflict. Here, it is the armed conflict during World War II.

Here, pour peur des croix [by fear of crosses] is an allusion to the swastika that soldiers of the Third Reich wore on their uniform.

Interpretation

When general Alexander takes charge of italian armies,
King Victor-Emmanuel, driven away from Rome
and fearing the soldiers with the swastika, takes refuge in Brindisi.
However, because Mussolini undermined the monarchy,
He will abandon his charges to his son in 1944

History

At 52, [an old age according to Nostradamus' standards] british general Count Alexander of Tunis, takes charge in Italy when the italians surrender to the allies:

Germans occupy Italy They disarm the italian army

[P.222] The armistice was signed in Malta on board the 'Nelson', september 29, by Eisenhower, Alexander [chef vieux britannique] and Badoglio. October 13, Italy declared war to Germany...at 7.45, Badoglio made the announcement that Italy had capitulated.
[Translation - Les Canadiens en Italie - Lt. Colonel G.W.L. Nicholson]

The old monarch, is Victor-Emmanuel III, who lived from 1869 to 1947. He is 74 when the german army occupies Italy in 1943, which makes him a very old monarch [le vieux monarque] indeed. Vexed and angered, the fuhrer, by way of reprisals, orders the arrest of high dignitaries and even the king:
[P.59] As early as tomorrow, Hitler exclaimed in front of his generals, I will send someone to Rome with orders to arrest all the government, the king, the heir to the throne, and all the rest of them, then Badoglio and his crew.
[Translation F.L. - Mémorial de la Seconde Guerre Mondiale, Tome 3, Sélection du Reader's Digest 1965 ]

Fearing the reaction of the Germans to the armistice, the king decides to leave Rome:
[P.443] Yesterday, Victor-Emmanuel III, [le vieux monarque]members of the government and chiefs-of-staff of the three armies left Rome for Brindisi. [par peur des croix ployera son enseigne]
[Translation F.L. - Chronique de la Seconde Guerre Mondiale - Jacques Beaugrand]

Victorr-Emmanuel III was forced to abdicate after the war:
He refused to in tervene when the fascists took power in 1922, and his acceptation of the fascist dictatorship became too heavy for the monarchy that could not be saved in spite of the major role Victor-Emmanuel played in the downfall of Mussolini [Le plus grand chef faillira au besoin] in 1943. He transfered his powers to his son [par conflit roi règne abandonnera] in 1944 and abdicated in his favor in 1946. (Encyclopédie Universelle Illustrée - Georges Pascal)