Xenophobia refers to an irrational fear of things that are foreign or strange. In other words, alien. Usually people use it to refer to ultra-nationalism and fear of other cultures and nations. But I want to discuss another use of the word.
Aliens! That's right. The irrational or unreasonable fear of aliens. If you've seen the movie Alien or its sequel Aliens, or Independence Day, or Predator, you know that irrational is not the right word. It seems to me that it is completely reasonable and rational to fear aliens after seeing xenomorphs, alien invaders, and invisible big-game hunters with weird dreadlocks and a built-in self destruct button. What's up with that?
Am I xenophobic? Am I afraid some intelligent lizard or a pod person is going to make me the main course at an alien buffet? No. Aliens don't exist. But xenophobia is a great plot line for any scifi novel. Without it, my entire trilogy would be stillborn. While it's not the main point of the novels, it plays a significant part in advancing the story line and adding tension. Without it, the entire conflict would have been resolved much quicker and easier. Good for the people fighting the war, bad for the reader who gets bored three chapters in and puts the book down. Therefore, xenophobia. Much better for everyone. Except the poor guys getting blasted by alien spacecraft. Sucks for them.