Foreign Policy Magazine, which usually publishes more mundane articles on the Iranian nuclear threat, the collapse of the Euro, or the rise of China as a super power, has a piece that tries to answer the question, are we prepared for an alien invasion?United States Air Force (public domain)
The article is primarily a feature about a number of weapon systems that are under development, likely directed at more terrestrial opponents than the title would suggest. They include laser weapons, reactive armor, and a variety of stealth vehicles.
In answer to the question posed, are we ready to repel an alien invasion; the answer is yes, if the aliens obligingly attack like those in the film "Independence Day" or the TV series "Fallen Skies." The American military, equipped with weapons such are featured in the Foreign Policy piece, as well as some likely too classified to be in the public domain, would likely give a good account of themselves, just like Will Smith and Noah Wylie in the two aforementioned dramas.
However, in keeping with Arthur C. Clarke's maxim that aliens may well have technology so advanced that it would be indistinguishable from magic, one wonders if aliens capable of crossing interstellar distances would attack with flying saucers and storm troopers in battle armor. Whether the movie is "Independence Day, "Battle: Los Angeles," or the entirely silly "Battleship," the aliens always have some weakness that the brave humans, Marines in the first two movies, and sailors in a World War II era battleship in the second, can exploit.
A somewhat more realistic scenario for alien invasion was depicted in the entirely excretable remake of "The Day the Earth Stood Still," with Keanu Reeves in the Michael Rennie role. The aliens in the remake propose to destroy the Earth using a "gray goo" cloud of nanites, little atom-sized machines designed to take apart human beings and all of their works on a molecular level. All the clever laser weapons, particle beams, genetically engineered diseases, or computer viruses that movie humans use to repel alien invasions would be useless.
Can one build a counter measure for a nanotechnology weapon? Hopefully, because as the ultimate weapon of mass destruction, a nanite-based device would be first rate. It may be able to destroy every bit of military hardware a nation possesses, leaving such a country defenseless to a conventional attack. It could wipe out every living thing in a target country, or even on the planet, along with infrastructure, leaving the Earth pristine, but lifeless. Perfect, therefore, for aliens to remake in their own image.