An alien invasion is an attempt by extraterrestrial life to invade Earth and conquer Humanity, although it could theoretically be applied to any species invading any planet. The concept usually appear in fiction and in hypothesis. Allegedly many different types of alien beings have visited Earth since the beginning of time, and though people have been attacked, killed and captured by these beings in the past, none have ever made an attempt to conquer Earth.
If aliens were to invade Earth some time in the near-future, it seems likely that Humans would not stand much of a chance in defending the planet from an alien attack because the invading forces would be much more advanced in technology and in weaponry, the Earth also has no orbital, or space defence systems (any known to the public anyway), while it is logical that any invading space-faring lifeform would have some sort of space weaponry. Though only fiction works have shown Human weaknesses in technology to invading alien forces, it is most likely that a technology gulf does exist, as any alien life-forms that have visited Earth in the past blatantly would have much more advanced space-travel technology at their disposal (otherwise how would they get to Earth?). In the Kelly-Hopkinsville encounter, it was reported that even the entities themselves seemed to be immune to bullets. In an alien invasion, a technology gulf and a lack of effective weaponry would easily prove to be Humanity's downfall.
If an alien invasion was to occur on earth, a cover-up by the governments of the world would not be possible simply because of the number of people who witness and are effected by it. An invasion would prove to the whole world aliens exist, although this would not be enough to make it a good thing.