The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
The article “the” is generally omitted with acronyms of proper nouns (NATO, NASA) but used with initialisms (the US, the UK).
Use “a” or “an” before an abbreviation depending on how it is pronounced not written. If it starts with a consonant sound, use “a”; if it starts with a vowel sound, use “an.”