Is the US ready to protect Planet Earth in case an alien invasion happens?
Sure, much in the same way that an ancient Stone Age tribe would be ready to protect each other in case a battalion of United States marines happened to show up with ill intent.
Look, any species that has the tech to invade us is going to be basically impossible to stop. Period. There is no defense against a species that has tech advanced enough to do what would be required to get here, much less do anything once they arrive.
The good news is that there's nothing they could get from Earth that they couldn't get more easily from just about any uninhabited planet anywhere else in the galaxy, so there's no need to worry.
It is troubling to encounter the viewpoint that only the U.S. can (or should) attempt to "protect" our planet, for whatever reason - including a hypothetical alien invasion. This smacks of hegemonic rule and imperialism. The military and defensive jurisdiction of every country, no matter how small or large, should be their own territory, and only that. Anything involving the welfare and protection of a large part of the planet should be organised by international agreement, involving all of the countries involved. So perhaps the questioner would like to rephrase the question in which "U.S." is replaced with something else, such as the UN (United Nations).
The Earth cannot be protected in that way. If extraterrestrials decided to destroy this planet, nothing could be done by humans to protect it. If we were concerned about this, we would never have began to communicate electronically. All of these transmissions are being carried into space, and I am reasonably certain that many of those communications have reached extraterrestrial lifeforms. If aliens had such a desire to irradiate life on this planet, it could be done without a single casualty on their part and the destruction would be total and complete. But I believe that they have no such desire, but instead would prefer to make corrections as needed.