The Cantacuzino brothers' political ascendancy came at the height of a Phanariote regime, when Princes were directly appointed by, and subservient to, the Sublime Porte. According to historian Neagu Djuvara, the Cantacuzinos were technically Phanariotes, but emphasized their matrilineal connection to the House of Basarab. As such, they "would soon come to lead a 'national party', one hostile to the growing influence of the Greeks". Pârvu III's first contribution to the Wallachian military forces came during Constantine Mavrocordatos' third reign (1735–1741). He was ordered to take up arms during the 1737 campaign against Russia; in fact, the army's role was to intervene between the Bucharest populace and the Ottoman army, policing against pillage and rape. This period inaugurated his participation in Russophile politics and intrigues. Appointed "Serdar" in April 1737, he signed his name to "Vornic" Preda Drăgănescu's memorandum which requested from Anna Ioannovna, Empress of All Russia, pleading with her to "liberate us [...] by any means". From June, the Habsburg Monarchy joined the war as a Russian ally, and invaded Wallachia, prompting the Prince and boyars into exile; several of those still present in Bucharest were arrested by Habsburg General Barkóczy, and deported into Transylvania. Pârvu and his father were among those who signed a letter of protest to the Russians, asking them to curb their allies' abuses. The document was successfully delivered by Dimitrie Scutari at Kiev—along with another memorandum which asked for Wallachia to be made a Russian protectorate.