At the end of 1973, Soviet Union conducted a series of unmanned missions to Mars, which were expected to reach the Red Planet in February and March 1974. Although the Soviet assault of Mars reached unprecedented scale -- four probes aimed the planet -- the campaign produced rather mixed results. None of the four spacecraft were able to complete their missions successfully, even though valuable data was returned.
The immediate impulse of the scientists in Russia was to prepare for the next launch window to Mars, opening every two years, and try again.
However, looking ahead at the 1975 launch window, the Soviet scientists had to consider one more factor -- the Americans. Although during this period, Soviet and US space programs had limited official interaction, they inherently influenced each other. Upon reviewing NASA plans for the Viking mission, scheduled for launch in 1975, Soviet scientists realized that at the present level of funding and the state of the Soviet technology, it would be very hard if not impossible to match the scale and ambition of the US project.
Rather then duplicating NASA efforts, Russia made a controversial decision to abandon immediate plans for Mars exploration and jump-start a series of missions to Venus, known as "Venera."