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.
I can see how you would think that - absolutely. BUT it wasn't consistent - there was no attempt at all at communication with Earth - and could equally be interpreted as they were the vanguard of a larger invasion force. But the Invasion motivation wasn't consistent either - especially the defensive approach and them being poop at fighting.
Meh - it was a sort of 'best without brains' type of film