Following the establishment of the dictatorial and fanatically anti-communist 4th of August Regime under Ioannis Metaxas in 1936, the entire Communist Party went underground.
From August 1941 until March 1942 he was first secretary of the prestigious Athens Party Organization (KOA), and played a major role in the establishment of the National Liberation Front (EAM) in September 1941 and in KKE's decision to launch an armed guerrilla campaign, leading to the establishment of the Greek People's Liberation Army (ELAS).
From then on, along with the "chief captain" Aris Velouchiotis and the senior military commander, Stefanos Sarafis, Tzimas formed the leading triumvirate of ELAS, with the nom de guerre of "Vasilis Samariniotis" (after his father's home town).
[7] In October 1943 he was sent as part of the first EAM delegation to Cairo for talks with the British and the royal Greek government in exile, and after December 1943 served as ELAS' liaison with Tito.
Despite his distinguished role in the Greek Resistance, after liberation he fell into disfavour with the party establishment: his failure to be elected to the Central Committee in 1945 was followed by his arrest and exile to Ikaria.
Although he escaped in 1947 and joined the fight of the KKE-backed Democratic Army of Greece in the ongoing Greek Civil War, he remained on the sidelines.