A person who sneaks into a country

One word substitute is Infiltrator.

Infiltrator: a person who enter or gain access to (an organization, place, etc.) surreptitiously and gradually, especially in order to acquire secret information.
Sniper: a person who shoots from a hiding place, especially accurately and at long range.
Invader: a person or group that invades a country, region, or other place.
Aggressor: a person or country that attacks another first.