Country

It is an expression mainly used in military or scientific contexts.

In-country:

being or taking place in a country that is the focus of activity (such as military operations or scientific research) by the government or citizens of another country:

• scientists and in-country colleagues will carry out field research

• Tropicus Conservation International readied my gear for my first mission in-country — Darryl Young

(M-W)

In-country(adverb & adjective):

In a country rather than operating from outside but in relation to it.

• as adverb ‘the people we're putting in-country will get instructions from satellite radios’

Usage examples:

‘And then it hands over management of its own in-country operation to that very same company.’

‘The training has begun to have a multiplying effect with initial in-country training now extending to courses conducted outside Iraq.’

‘Typically the station operates out of an in-country US installation, with or without the knowledge of the host country.’

(ODO)

As shown in Ngrm the expression was used from the early '60s, probably during the Vietnam War as suggested in the comment from (answers.com)

• Since the war lasted so long and so many men were rotating in and out of South Vietnam from so many branches of service; US Army, US Marine Corps, US Navy, US Air Force, US Coast Guard...and probably civilians too (CIA, Contractors, etc.), plus there were so many 2nd, 3rd, and more "tours" in the Vietnam war...it simplified everything all the way around to simply state "in country." Meaning "he's either here or he ain't."