Go into this movie, not rooting for the humans, but imagine the aliens as a Federation species attempting first contact with a communicating species... Trust me, you'll like it.
HERE BE SPOILERS
This 'invasion' is clearly nothing of the sort. It barely qualifies as a military expedition. This is clearly a diplomatic, first contact mission gone horribly, horribly wrong.
-When they land, they don't go for population centers, manufacturing hubs, or other militarily or logistically significant targets. They land at the site of the communication. This is what I would expect from someone I invite. To show up at the 'door,' not raid my garage.
-Throughout the movie, their standard ROE seems to be matching scales of intensity. They don't attack clearly military facilities until they are directly attacked, even going so far as to disengage from combat with proven hostiles. They show considerable restraint in this self-defensive action, and NEVER deliberately target civilians, even going so far as to inconvenience themselves to avoid it.
-Their primary mission throughout the movie is to get to the communications array and contact home. Considering the welcome, this is probably an SOS or quarantine order.
-Their initial contact is bog-standard first contact. The ship communicates with a sound, so do they. It isn't their fault it's too loud. They move to protect one of their own that's being fired upon, and specifically target to disable weapons when they do open fire. It's clear throughout that they don't want this conflict.
-Their weapon loadouts are completely insane for a military invasion. They have two displayed weapons: A short-range, non-guided sticky-grenade and a long-range, intelligent, precise roving ball (assuming the initial blast thing is in fact their equivalent of a horn). This is not what I would expect from an invasion, but it is what I would expect for a 'just in case' security escort for a diplomatic envoy; a crowd-control weapon, and something to take out snipers.
-That an interstellar vessel can be utterly destroyed by impacting one of our satellites (which are very fragile in and of themselves) displays a stunning lack of armor. Since it was their communications ship, this would be their command vessel in an invasion fleet. It should be fairly well-armored. That it is not so suggests that it is, in fact, either a diplomatic ship or a completely civilian vessel.
-The one and only unprovoked attack on human settlements is clearly-executed with a mission in mind. They are disabling the military inside the shield, preventing them from getting to the communications array. They disable the main highway for ground transport to the mountain, incurring minimum civilian casualties. Their second target is the aircraft of the hangar. The only people we see dying in that attack were shooting at the spheres. In fact, the spheres notably ALTER their course to avoid fleeing military personnel.
Ergo, they're clearly following a very Federation stance on their interactions. It's a fantastic Star Trek movie (if you can get over the fact that the Saurian vessels seem radically divergent from what you'd expect).