国 (traditional 國) is a treasure inside walls. The outer box 囗 is a border or city wall; inside the simplified form sits 玉 (jade) — the king's most precious possession. A walled space protecting what's precious = a COUNTRY. The traditional form held 戈 (a weapon) defending the borders instead.
Now you can finally say 中国 — China, the 'Middle Kingdom' — the phrase 中's lesson promised you. 国 builds nation words everywhere: 美国 (America, 'beautiful country'), 英国 (Britain), 法国 (France), 外国 (foreign country). 国人 means one's countrymen.
Jade (玉) locked inside city walls (囗) — a precious COUNTRY.
Other countries are named with 国 too: 美国 (USA, 'beautiful country'), 英国 (UK), 德国 (Germany). The character you pick before 国 captures how Chinese first heard each nation's name.