If the aliens target the entire Earth, then Earth would probably become united. It seems like all people have some sort of a nemesis - lets call that the Skull-King (http://www.Smbc-comics.Com/index.Php?Db=comics&id=2244)
Some bigoted Christians have gays as their Skull-King, some bigoted atheists have religion as theirs, extreme feminists consider men their Skull-King, PETA are opposing meat-eaters, etc.
If the aliens came and attacked the whole Earth, they would become the Skull-King who will unite the nations of the Earth to fight against them.
But the truth is that this would also be very temporary. As soon as the threat is destroyed or becomes not as imminent, people will start hating each other again, and competing nations will turn on each other. Only if the war goes for a very long time will people forget their past differences with other nations, because all they'll grow up to hate will be the Skull-King... But even then, new hatreds will eventually be bred. Mankind never has, and never will change.
I don't know why Bill Clinton is talking about aliens, but his idea could work. Sure, it depends on the species and their goal, what they do to us, and their technology, etc., but it definitely could bring more peace to the world when they're gone. It shows that there is still good in the world somewhere when people unite under a common enemy.
If the earth were to be attacked by an alien civilization hellbent on wiping us off the face of our home planet, you can be assured that people would come together then and the differences between countries would seem pathetic. We would have no choice but to unite and fight together.
If an alien invasion happened the world would have no chance but unite its collective resources. An external enemy would create a sense of worldly nationalism, and suddenly rather than being nationality vs nationality we are all just people. However, if an alien invasion did happen they would likely be so advanced that resistance would never come to fruition.
We live in a world where races, ethnic groups, nations and even religions hate each other. It would take an alien invasion to get us to unite and put aside our difference and fight off a more vile and violent threat. So yes, I believe what Clinton says this one time.
From what I have read of President Bill Clinton's alien comments, he's just speaking hypothetically and doesn't think this type of experience is imminent. But if aliens did attack the Earth, not as friends, but with ill intent, it would make sense for all nations to join together against a common enemy toward a common goal.
While it's rather morbid to hope for an alien attack to unite the world, I do believe that it would have that effect. Looking back on past world wars, and seeing how different nations united together in order to protect each other from the greater good, an outside force that threatens everyone equally would have just as much chance of uniting people.
The only way this world will ever have peace in my opinion is if we had to defend from an outside threat because it would be in everyones interest to protect the earth. There may be instability and caution to each other at the start but when people started dropping like flies everyone would realise what needs to be done.
If aliens ever did attack, we would all be dead. When you look at every living thing on this planet from a germ to human, they all do the same thing when new territory is available. They carve out a niche, gather resources, and eventually take over as much area as they can. If we as humans have not advance beyond that, what makes you think aliens would be any different. Seeing that the closest planet to our own that could sustain life is light-years away, that would mean that they have advanced so far above us that they could make the trip in a very short amount of time, or they traveled a long amount of time in order to get here. The farthest we have sent people is to our own moon and back. That was back in the late 60's early 70's. What was one of the first things we did after landing? Plant a flag and claim it. Imagine how much more advance we would need to be in order to even get to a planet light years away much less send people. By comparison, we would be cavemen to the aliens. What purpose would a much more advance people have with cavemen? To take over, either because they wanted more planets to live on and use as bases or because they trashed their planet worse than we have trashed ours.
The other problem movies have is how aliens would attack the earth. They send down troops and are either somehow killed, or they are defeated by humans. First off, they would not just send down troops. When was the last time an advanced nation sent in troops as their first move? Usually, we bomb the daylights out of an area, send missiles to destroy key targets, by the time we actually send people, they land on a large chunk of land almost void of all people. Before they move from that area they bomb even more. We clear land, then send troops. What makes you think an advanced people would not do the same if not worse. If they are that advanced, they would probably have a weapon to destroy all humans if not all life on the planet from the safety of their space ship. It would probably be all over for the entire planet in less than an hour. If we are lucky, they will allow some to live as pets or slaves. If the pets or slaves revolt, they would put us down in a heartbeat. Sure, we would all be united, we would all die at about the same time, so we would all die as one.
If people knew the truth about ET's, the societies they have, and their purpose in the Universe than people would be pissed/blown about the false misconceptions, paranoia, and beliefs that some governments have created for our societies. We have come to believe that if we were visited by an advanced extra-terrestrial civilization they would want to destroy our beautiful planet; this would've happened already. Why do you think that when the government acknowledges a "UFO" the "UFO" disappears? It is because these 'higher' beings do not want any conflict or destruction - we have already destroyed the planet enough - they just want to observe the current state that Earth's humanity is in. They are not all Grey's, most of them are similar to us, movies just created a false perception. These beings want to preserve this beautiful planet because we are obviously not preserving it enough ourselves and for them to destroy our planet would be like the government ordering the mass murder of all poor people. There is no benefit in that and they are all purely benevolent (besides the Draconians) and loving towards us but most are too ignorant to believe this. In the near future we will be disgusted by the government when we make actual contact with a benevolent ET race and learn many actual Truths. The reason they haven't made contact with us is the fact that do you think with all the wars America makes that we are just going to kindly accept them onto our planet? No, most will panic and assume the worst then they will attack, leaving a scar on a potentially beneficial relationship. It's like if your friend offered help with studies, you get mad for no reason and punch him in the face, and he never wants to talk to you again. Once everyone understands how helpful these beings can be then they will publicly show themselves with the hope of not getting ram-sacked.
The aliens could get support from humans. And maybe not entirely unwarranted support if the aliens promised a significantly better world. It's not necessarily the case that invading aliens would want to wipe us out or even enslave us. Aliens could come and invade with the same sort of intentions the US had in Iraq. They may want to effect a regime change according to their ideology on a planet that conveniently has resources they want.
The aliens could also just pick on some countries. Maybe they are supporters of democracy and come and invade Russia and China and the United States decides it supports what the aliens are doing. Or maybe the aliens will have a one-party system and semi-planned economy and invade the US and China will approve of the invasion. The aliens could also wet nations beaks, granting trade deals, access to technology, and large amounts of what ever currency is used in intergalactic trade.
In fact I'd expect aliens wouldn't invade militarily. With all that power why bother? They would get power with business contracts. Maybe there would be an intergalactic EU and we'd have to agree to certain terms in order to join.
As with any conflict, there are those who look out only for themselves. If aliens invaded, there would be many who would choose to support them. A traitor is a traitor. Period. I'm sure the aliens would be able to bribe many into supporting them. Human nature never changes. History teaches us that, if nothing else.