John Brandenberg said that two Martian civilizations known as the Cydonians and Utopians were absolutely wiped out due to attack from an unidentified alien race. He further proved his claim by giving evidences such as the numerous nuclear isotopes on Mars' atmosphere is from the hydrogen bombs that Earth releases.
"The high concentration of Xenon-129 atmosphere, the evidence from Krypton-80 and the detected pattern of excess abundance of Uranium and Thorium on Mars surface, relative to Mars meteorites, first seen by the Russians and now confirmed by the Mars Odyssey Spacecraft Gamma Ray Spectrometer, mean that the surface of Mars was apparently the site of massive radiological events, which created large amounts of signature isotopes and covered the surface with a thin layer of radioactive debris enriched in certain elements relative to its subsurface rocks. This pattern of phenomenon can be explained as due to two large anomalous nuclear explosions on Mars in the past." - John Brandenberg
Brandenberg stands by his beliefs that there is indeed life on Mars and how it got vanished for years. The civilizations he pointed out as Cyndonians and Utopians is very reminiscent to the ancient Egyptians, only advanced in a way or two. The reason for those civilizations' vanishing was due to the area and land on Mars that was particularly fueled the fire for the other civilization to target and get rid of.
"The existence of a dead humanoid civilisation on Mars is completely consistent with its apparent long-lived Earthlike past climate and the Principle of Mediocrity, the idea that humanoid intelligence is not exotic or miraculous, but is a natural consequence of any long lived Earthlike biosphere. This Martian civilisation apparently perished due to a planet-wide catastrophe of unknown origin that changed Mars climate from being Earth-like to its present state in a brief period compared to geologic time. However, what ended this civilisation, was it a massive nuclear attack?" - John Brandenberg
Another reason for other civilization to get rid of other civilization is due to a belief that those civilizations poses as a big threat to older civilizations with their more advanced intelligent life. So it is quite normal for older civilizations to attack and wipeout their younger civilizations in hopes that they won't get replaced.