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It was a long time ago, so I remember only some particular.
The alien was something like an insect, and they were greater than human. The human was taking a bomb to the alien's home planet (I think), but somehow the alien get into the human ship and I remember the ending scene:
one man has a bionic eye, and looks for targeting the enemy world on a rod-like connection, but one of the man was a particularly small alien, and kills him pushing its head in the rod. The other man manage to pull the trigger, but in the end the alien wins, as the earth was on target