Final answer:
The claim that both the Soviet Union and the United States proposed missions to Mars after WWII but did not send humans is true. While there were ambitions and developments for manned missions, only unmanned missions were launched, with some failing before reaching Mars.
Step-by-step explanation:
The statement that after World War II, both the Soviet Union and the United States proposed missions to Mars but neither sent a manned spacecraft there is true. During the space race that followed WWII, both powers indeed had ambitions for Mars exploration and even developed technology for potential missions. However, no manned spacecraft have ever been sent to Mars by either country. Instead, the rivalry resulted in a series of unmanned missions that provided valuable data about the Red Planet.
After the Viking missions, Mars languished unvisited for two decades. Later attempts by both NASA and the Russian Space Agency to reach Mars did result in spacecraft launches, but unfortunately, both of these missions failed before reaching the planet. Adding to the complexity of space exploration, current technologies and political will have not yet aligned to send humans to Mars or even back to the Moon, though there's consistent scientific interest and unmanned missions that continue to explore our celestial neighbors.