In the extremely rare case where life, and intelligent life, is appearing on two distinct planets in the same solar system* during a relatively same time-frame — a conflict, a war, or even a colonization scenario may become extremely probable in function of the technological and social advancement separating both civilizations.
Said otherwise, if life on Mars survived until now (see here for ancient Mars habitability proofs), what would have happened ? Would they have come on Earth ?
An artwork from the film Avatar (2009), where Humans meets Na'Vi.
* See the Golden Belt or Green Belt or Goldilocks zone for habitable zones inside solar systems — or the zone between the minimal and maximal distances where the stars's light exposition is right for life to emerge (as understood in 2024 — and depending from the sun type and mass).