Hawking should stick to physics and math. Any intelligent life sufficiently advanced to travel interstellar space will not require the resources of biological life.
I don't think he's talking about "they'll show up in warships and star a war." I think he's saying that the may show up in peace, but in the long run our civilization will probably be overrun by them." When Christopher Columbus showed up in the West Indies, he was very friendly. Now native americans live on reservations after having everything taken away from them over the last several hundred years.
Spain was not kind at all with the Aztecs, Europe enslaved the black race from Africa...there's too many examples from our own history. It's a reasonable assumption to say that contact with a high tech race will spell the decline of the lesser tech race.