In 1968 he founded the Maoist group "Alle macht aan de arbeiders" (All Power to the Workers), which in 1979 became the Workers' Party of Belgium.
Martens wrote on the political history of the Democratic Republic of the Congo, where he has lived and travelled extensively.
In 1994, Martens published Another View of Stalin, a history of the Soviet Union under Stalin that challenges in particular the dominant view of collectivisation in the USSR and the Great Purge. He explained his motivation for writing the book in the introduction: Defending Stalin's work, essentially defending Marxism-Leninism, is an important, urgent task in preparing ourselves for class struggle under the New World Order.
Martens writes primarily in French; however, his books, especially Another View of Stalin, have been translated into Dutch, English, and numerous other languages. He is a leading Marxist theorist within the anti-revisionist movement, and is therefore perceived as a "Stalin apologist" by the dominant bourgeois intelligentsia.
In Another View of Stalin, Martens regards as the main factor behind the Ukrain famine (Holodomor) to be bad conditions and the class enemy. With regard to Aleksandr Solzhenitsyn's research Gulag Archipelago, Martens stated: This man became the official voice for the five per cent of Tsarists, bourgeois, speculators, kulaks, pimps, maffiosi and Vlasovites, all justifiably repressed by the socialist state.
Within the International Communist Movement, he is noted for having proposed the unification of the four main tendencies of the Marxist-Leninist movement. These are the pro-Soviet groups, the pro-Chinese, the pro-Albanian, and pro-Cuban. In addition there are "independents." Martens has put forward that while at a certain time these separations were important and based on principle, they can now be overcome and the movement can be united on the basis of Marxism-Leninism. In order to develop this unification process, the Workers Party of Belgium hosts the International Communist Seminar in Brussels, which is attended by 150 organizations around the world. According to Martens, "Today, as a result of the restoration of capitalism under Gorbachov, the "pro-Soviet" tendency crumbled into innumerable tendencies. In the sixties, a "pro-Chinese" tendency emerged but split into various tendencies after Mao's death. There has been a "pro-Albanian" tendency, which also split after the collapse of socialism in Albania, and a so-called "pro-Cuban" tendency, mainly in Latin America. Some parties, finally, maintained an "independent" position vis-a-vis the tendencies mentioned. Whatever one's opinion about the correctness or the necessity of these splits at a certain point in history may be, it is nowadays possible to overcome these divisions and to unite the Marxist-Leninist parties, which are divided in different currents."