The text says, "Death to World Imperialism", and Google tells me the poster was published in 1920. The snake is imperialism, wrapped tight around an industrial city representing the global proletariat, but is being battled by the courageous Russian people.

This was just after the Revolution, when Russia was still in the midst of a brutal civil war between the Soviets and anti-communist forces collectively known as the White Army. Various western powers, including Britain, America and France, as well as Japan intervened on the side of the Whites and invaded Siberia and Northwest Russia between 1918 and 1920. So at the time the poster was published the Soviet Red Army was in a very really war against what they considered "imperialist" powers.

There's also a broader context: orthodox Marxist thought says that the only successful socialist revolution is a global one. The Bolsheviks still held to that philosophy, and expected their revolution to spark proletarian revolts elsewhere. So, even though they'd thrown off the old imperial autocracy of Russia, they saw themselves as being engaged in a larger struggle against capitalist/imperialist governments all around the world.