In 1930 he was promoted to full colonel, serving as military attaché to Yugoslavia and Czechoslovakia, Director of the General Staff's Second Bureau (Intelligence) and as Deputy Commandant of the War College.

[4] During the Axis Occupation of Greece, he founded the RAN resistance group,[2] but in 1943 he fled to the Middle East and rejoined the Armed Forces of the Greek government in exile.

With the rise of the Communist-controlled National Liberation Front to a dominant position in the Greek Resistance, Ventiris, like many other Venizelist officers, increasingly moved to right-wing and pro-monarchical views.

[4] Despite his ardent anti-communism, as an ex-Liberal he was a rare figure among the monarchist army leadership, and was often the target of right-wing newspapers for his former Republican views.

[3] Thereafter he served as Deputy Chief of the General Staff, and with the outbreak of the Greek Civil War he was appointed CO of III Army Corps.