OP here, i'll elaborate on my scenario. Assume that the aliens and humans have a similar level of intelligence, similar level of technology and have a similar level understanding of scientific concepts overall. They do not have the exact same science as each other, but in general have a level that would mean if both races decided to have a war with each other, the damages suffered by both sides would be the same as if the damage was caused by their own race.

In order for the alien race to have come into contact with humans, it is assumed that the aliens have technology high enough to travel through space with ease. In this case we could assume that this scenario is based sometime n the future, where humans have a similar level of technology (but have not found the aliens, "by chance".)