So how does humanity react to a fleet full of aliens pulling up in orbit? How does the UN react? What do the leaders of major countries do? How do religions world wide deal with this new revelation?
Humanity acts, as everyone else has said in a mix of emotions. Some will panic. Others will be excited. Personally I will see it as the greatest moment in human history but I will be very cautious and a little sad. It will more than likely mean the end of our culture. If the fleet is in orbit I will also leave the city.
UN will be basically useless for several weeks as the countries strive to see who deserves to speak with the aliens (That's assuming they follow the first contact plan Rochey describes). I think eventually they'll be able to have meetings but we don't have a world government. However I think this could eventually turn us into a world government.
I'm going to take the opposite view of most of you. I think religion will flourish again, at least in the short term. People will return to God to attain the answers they desire. New religions will form up yes. And if the visitors have a religion expect that to come HUGE.
1) They mention that there's about a half dozen other races of varying power and size in this part of the galaxy as well.
A world government would become an all but guarantee. I think humanity will only unite if there are outside pressures.
2) They're at war with one of these races.
Earth would want to stay neutral of course but as others said we would want to know more information. I think we would be a little guarded with access and wouldn't allow a colony on Mars at least in the short term.
3) They look like horrific monsters from the darkest depths of our imagination.
Hopefully they would realize this and visual contact would be held off. However that would make people paranoid. I think if its done slowly, as not to 'spook' anyone it might be okay. And remember looks can also help determine what kind of creature it is. A predator would have eyes on the front of its head for example. If the creature looks like some horrific monster we might -want- to be cautious.
4) They're a highly militaristic society under an authoritarian government.
This would be a cause to worry. At time of first contact we would not be able to do anything about it. However more technologically advanced cultures always erase less technologically advanced cultures. It might be VERY bad news for Democracies and Republics.
5) Something goes horribly wrong when they turn up, like one of their ships FTLing in and smashing the ISS, or a shuttle losing power and leveling the White House in the crash.
Huge anger. If it took out an entire city it might be best if the aliens just left at least for 10-20 years. However if it took out the White House.. say when Bush was president I personally would be upset with the loss of historical pieces but that would be a forgivable offense.
6) The aliens are an incredibly pacifistic race (so they've no warships in this case) and start preaching about how violent and barbaric we humans are. They say that they'll be happy to trade tech with us, including starship technology, but only if we disband our militaries and get rid of our nukes.
We wouldn't take it. Nor should we. We cannot trust them and if there are other races out there we shouldn't trust those either. While our weapons might be incredibly inferior eventually we will catch up. Having the right to defend ourselves is important if we want to maintain sovereignty and not be a protectorate.
A few months after first making contact with us (assuming they haven't been told to bugger off or haven't nuked us all yet) the alien fleet's leader comes back with a request. It seems that the Earth is about half-way between two of their planets, and they'd like to construct a large space station in orbit to use it as a refueling depot for their freighters. Does humanity go for it, or tell them where they can stick their station?
I agree, build it over Mars. Also Earth must receive technological advancements in return or perhaps a share of the precious minerals. Its a shame its Mars though. If it was Mercury that would be a lot better for humanity
And a final scenario. It's about a year since first contact and a few Earth nations start receiving a few odd requests. It seems that some of the aliens quite like Earth, and want to live here permanantly. Would countries actually let them live here? If so, how would the populace react to the chance of having an alien move in next door?
I think some countries would however they should go through all the other avenues Earth aliens go through to become citizens. I want my aliens to know the bill of rights damnit-- especially if they're part of a totalitarian government.
As far as the US screwing things up... I actually think the United States is probably one of the best prepared for an alien encounter. Rather or not aliens have been contacted with in the past its been taken as a serious enough threat for there to be military plans on how to deal with an alien invasion. Hell even Reagan, the original Neocon (well maybe Nixon was that), was in favor of a united world response to an alien encounter. Why? Corporations exist to make money. Aliens are a huge threat to our culture. Never in history has the less technological culture survived. That is a HUGE uncertainty to corporations. So even the most staunch neocon of a president, like Bush Jr would be more capable of handling visiting aliens than say France. And the more progressive presidents are cool headed enough for that not to be an issue. I can't imagine Obama doing anything but a cool and collected response.