The 'Green House'

Well, first of all, I don’t think that it would be possible for an alien to land on the White House lawn. The White House has extensive security measures, and surely a spaceship would set off anti-aircraft missiles and provoke sniper fire. The only way it would be possible for an alien species to land anything on the White House lawn would be if
a) The alien had anticipated and had in-depth knowledge of White House security measures and had taken preventative measures, such as disabling the defenses or becoming invisible, or
b) The aliens’ spacecraft and the aliens themselves were heavily armored and able to withstand the attack.
However, assuming that the aliens were indeed able to land on the White House lawn, they would probably be met with considerable apprehension. The first thing that they would have to do would be to assure us that they come with peaceful intent. Since the aliens had the technology to withstand our defenses, it is likely that, if they had desired to hurt us, it would have been possible, and easier for them to do so from space.
When the United States government had established that the aliens were friendly and had found a means of communications, the first move would likely be an exchange of technology. We would definitely want the aliens’ technology and knowledge of the universe, while we may have customs, knowledge, or technology that the aliens wouldn’t be familiar with. This would work because even though the United States would be getting more from the transaction, we still wouldn’t be able to bring our technology up to the level that would threaten the alien’s sense of security. This kind of positive-sum gain would be beneficial to both parties.
As time went on, it would be inevitable that the rest of the world would find out about our contact with extraterrestrials. Interaction with the aliens was governed by liberalism, with mutual gains benefiting both parties. However, in dealing with other countries, we would have more of a constructivist viewpoint. Since we clearly would have the upper hand in dealing with other countries, or identity rather than our sense of security would govern our actions. The historical role of the United States as a protector of everything good and just in this world (ok, maybe I shouldn’t take it that far…) would dictate our relations with other countries. Although we would send aid, it is doubtful that we would share our newfound technology.
Many other countries, especially our enemies, would feel threatened by our superior technology, but we would enjoy having the upper hand in the world and our return to the status of the world’s only hyper-power. This feeling of insecurity would force other countries to beef up their own security and take defensive measures. Most likely we would be able to contain any security threats, but in the event that we had half the world against us, we probably wouldn’t hesitate to take out a select few countries. This could result in a world war, ending the world as we know it.