Where in the world?
The flag of Cambodia has a blue field with a red stripe in the center and a white depiction of Angkor Wat in the middle.

Cambodia

Kâmpŭchéa

Population: 16,718,971

Region: Asia

Subregion: South-Eastern Asia

Capital: Phnom Penh

Top Level Domain: .kh

Currencies: Cambodian riel

Border countries

The flag of Laos has three horizontal stripes of red, blue, and red, with a white disc in the center of the blue stripe.

Lao People's Democratic Republic

Population: 7,275,556

Region: Asia

Capital: Vientiane
The flag of Thailand has five horizontal stripes of red, white, blue, white, and red with the blue stripe being twice as wide as the other stripes.

Thailand

Population: 69,799,978

Region: Asia

Capital: Bangkok
The flag of Vietnam has a red field with a large yellow five-pointed star in the center.

Vietnam

Population: 97,338,583

Region: Asia

Capital: Hanoi