He notably devised a plot to overthrow Carol II, King of Romania, a plan that would be taken over by Legionary leader and later Vice President of the Council of Ministers Horia Sima.

Cantacuzino was killed on September 22, 1939, at the prison in Râmnicu Sărat, during a retaliation operation ordered by Carol II following the assassination of Prime Minister Armand Călinescu.

Elena Lupescu, the Jewish mistress and later wife of Carol II, was specifically targeted at this congress, alongside Virgil Madgearu, General Gabriel Marinescu, and later Prime Minister Armand Călinescu, whom Legionary students believed to be conspiring to assassinate Codreanu.

[5] In 1936, Cantacuzino left with the legionary team led by General Gheorghe Cantacuzino-Grănicerul to fight in the Spanish Civil War on the side of the Franco-led Nationalist faction.

It was later intended to be transformed into the elite defence corps of the Legionary movement, "a group of strictly and severely educational essence and heroic inspiration", consisting of 10,033 "fighters" under 30 years of age.

The intended transformation of this organization was never realized, as the Iron Guard, the electoral front of the Legionary movement, was disbanded with the establishment of a royal dictatorship and the prohibition of any form of political activity.

Cantacuzino was arrested but managed to escape together with Vasile Cristescu, jumping from the window of the train transporting them from the Miercurea Ciuc camp to Jilava Prison.

Thus, within a few months following the April 1938 wave of arrests, Legionary nests were able to organize under conditions of secrecy, launching a number of manifestos and publications against Armand Călinescu and Prime Minister Nicolae Iorga, who they saw as responsible for the conviction of Codreanu.

[15][16] Publications such as "The Truth About the Trial of Corneliu Z. Codreanu, May 1938" and manifestos against the Royal Dictatorship were also printed, with Sima's Persecution Command directly addressing letters to the authorities.

He was imprisoned in the Râmnicu Sărat Prison along with the Legionary leaders convicted in the trial of 25 July 1938, namely Gheorghe Clime, Mihail Polihroniade, Traian Cotigă, and Șerban Milcoveanu, among others.

[17] The list of those to be shot was reportedly drawn up by Armand Călinescu, alongside police prefect Gavrilă Marinescu and Mihail Moruzov, the Romanian intelligence director.