In native pronunciation, however, nation has the long a of face, day and major, while national has the short a of trap, fashion and passion. Here are the two words, each followed by its first part:
So nation rhymes with station, location and situation (in fact with all words that have the suffix -ation), while national rhymes with rational, irrational – and not much else. You might say it’s quite an irrational pronunciation.
Note that ration also has short a. There are only three common 2-syllable words ending -ation, namely nation, station, ration, and ration is the odd one out: /ˈraʃ(ə)n/.
As Eric Armstrong (@voiceguy on Twitter) points out, American English allows ration to be pronounced with the short a or, alternatively, with the long a like nation and station. The short variant /ˈræʃən/ is the first or only variant given in the American dictionaries I’ve consulted.