Stalin adhered to the Leninist variant of Marxism. In his book, "Foundations of Leninism", he stated that "Leninism is the Marxism of the epoch of imperialism and of the proletarian revolution". He claimed to be a loyal Leninist, although was—according to Service—"not a blindly obedient Leninist". Stalin respected Lenin, but not uncritically, and spoke out when he believed that Lenin was wrong. During the period of his revolutionary activity, Stalin regarded some of Lenin's views and actions as being the self-indulgent activities of a spoiled émigré, deeming them counterproductive for those Bolshevik activists based within the Russian Empire itself. After the October Revolution, they continued to have differences. Whereas Lenin believed that all countries across Europe and Asia would readily unite as a single state following proletariat revolution, Stalin argued that national pride would prevent this, and that different socialist states would have to be formed; in his view, a country like Germany would not readily submit to being part of a Russian-dominated federal state. Stalin biographer Oleg Khlevniuk nevertheless believed that the pair developed a "strong bond" over the years, while Kotkin suggested that Stalin's friendship with Lenin was "the single most important relationship in Stalin's life". After Lenin's death, Stalin relied heavily on Lenin's writings—far more so than those of Marx and Engels—to guide him in the affairs of state. Stalin adopted the Leninist view on the need for a revolutionary vanguard who could lead the proletariat rather than being led by them. Leading this vanguard, he believed that the Soviet peoples needed a strong, central figure—akin to a Tsar—whom they could rally around. In his words, "the people need a Tsar, whom they can worship and for whom they can live and work". He read about, and admired, two Tsars in particular: Ivan the Terrible and Peter the Great. In the personality cult constructed around him, he was known as the "vozhd", an equivalent to the Italian "duce" and German "fuhrer".