word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."

A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.

"of or pertaining to a nation or a country regarded as a whole; established and maintained by the nation; peculiar to the whole people of a country," 1590s, from French national (16c., from Old French nacion), and also from nation + -al (1). Opposed to local or provincial (or in the U.S., state).

Meaning "peculiar or common to the whole people of a country" is by 1620s. From 1802 as "established and maintained by the nation or its laws." As a noun, "citizen of a (particular) nation," from 1887. Related: Nationally. 

National guard is from 1793, originally in reference to an armed force in France identified with the revolution; U.S. use is from 1847, originally a name sometimes given to the organized militia. National anthem is recorded by 1806. 

A King though he's pestered with cares,

    For the most part he's able to ban them;

But one comes in a shape he never can escape—

    The implacable National Anthem!

[W.S. Gilbert, "His Excellency," 1894] 

1851, from international + -ism. Related: Internationalist.

1860, with reference to law; see international + -ization.

1864, from international (adj.) + -ize. Related: Internationalized; internationalizing.