Why environment variables aren't being passed in a single command

Question

In bash, I can pass an environment variable to a single command in the following way:

KEY=VAL <command>

However, I don't understand why the following doesn't work:

KEY=VAL echo $KEY

While this works:

KEY=VAL bash -c 'echo $KEY'

i.e. the first one prints a blank line while the other prints "VAL". I'd expect both to print "VAL".

Answer 1

Because KEY=VAL echo $KEY isn't having echo expand the $KEY variable.

The current shell is doing that before it runs echo (or whatever).