His best-known novel is titled Hasta La Vista and recounts his experiences as a volunteer of the Republican forces during the Spanish Civil War.

[2] In August 1936 he joined the Garibaldi Battalion of the Republican forces of the Spanish Civil War along with other notable Albanians like Mehmet Shehu, Asim Vokshi, Emrush Myftari and Thimi Gogozoto.

[1] His best-known work Hasta La Vista published in Tirana in 1958 was largely influenced by his experiences during the Spanish Civil War.

In a letter sent to the Supreme Military Court of Albania from prison, he would state: Petro Marko is sentenced to three years and is thrown into the midst of those who he fought against, and hated.

For me, this is the greatest injustice, it is a crime that takes place against me, because I am innocent and never ever, starting as early as 1932, did not bring any harm to the people.... Another of his letters sent to the Supreme Court, still while serving time in jail, he would mention that he had been tortured and forced to admit something, otherwise he would die.

[4] Many works of Marko exhibit surrealist motifs and patterns such as his novel Qyteti i fundit [sq] (English: The Last City), portraying the end of the Italian occupation of Albania.