Where in the world?
The flag of Western Sahara has a horizontal tricolor of black, white, and green with a red triangle on the hoist side.

Western Sahara

الصحراء الغربية

Population: 510,713

Region: Africa

Subregion: Northern Africa

Capital: El Aaiún

Top Level Domain: .eh

Currencies: Moroccan dirham

Border countries

The flag of Algeria has two vertical bands of green and white, with a red crescent and star centered over the boundary between the colors.

Algeria

Population: 44,700,000

Region: Africa

Capital: Algiers
The flag of Mauritania has a green field with a gold star and crescent in the center, and two red stripes at the top and bottom.

Mauritania

Population: 4,649,660

Region: Africa

Capital: Nouakchott
The flag of Morocco has a red field with a green pentagram in the center.

Morocco

Population: 36,910,558

Region: Africa

Capital: Rabat