Let's reflect a moment...
1) There are a non-trivial number of them in our country (13 million or more)
2) They have entered the country illegally and remain here illegally.
3) A small (but significant) portion of them are involved in other forms of illegal trafficing.
4) A small (but significant -- and growing) portion of them are involved in illegal gang activities and violence toward US citizens.
If you don't like "occupiers" what term should we use? "Tourists", maybe?
P.S. And don't use France in WWII as an example. The French would let anyone do anything as long as they don't have to fight.
"Hostile Occupation" is a term of art. Laypersons see the term as one that is open to various interpretations, but it has a very specific and set meaning in the legal world.
Your attempt to classify them as "hostile alien occupiers" is flawed in that you're missing the "military" element, and the "exhibiting authority and control over the territory" element. In order for them to be appropriately classified as hostile alien occupiers, they would actually have to forcefully usurp the government, or we'd have to come to some agreement with them where they are granted "authority and control" over a US territory (like France did with Germany).
The correct term is not tourists, obviously, as "tourists" are individuals admitted to the US as a nonimmigrant visitor for pleasure, pursuant to INA § 101(a)(15)(B).
The term previously preferred was "illegal aliens," but that has fallen out of favor with many in the legal world because it is ambiguous and imprecise. It does not, for example, speak to whether the individual is one who entered the country without authorization, or one who entered the country lawfully but then overstayed. It also causes problems, because some people who are considered "illegal aliens" might not currently have the proper documentation, but might be eligible for asylum or other legal remedies.
My books use the term "undocumented migrant." That works for me.
You can call them whatever you want. But if you want to get in a legal discussion, you have to use certain words in certain ways. Lawyers are quite picky about that stuff.