They were a team of scientists, scouts and explorers on a diplomatic mission and in search for a refuge for their people, who were being slaughtered by an enemy on their planet. They misinterpreted the U.S.'s probe as a friendly response to their S.O.S.
The U.S. was in the wrong from the start. For sending a probe and for attacking the aliens when they came.
How do you know this?
I've seen it twice and I don't remember any of this.
I don't think I fell asleep during it