What is commonly referred to as Southeast Asia is in fact two distinct regions: the Indochinese Peninsula, which was named this way by Europeans because of the dual cultural influences of India and China, and the Malay Archipelago, a large group of millions of islands, which is by far the most populated island group in the World.
On this map, we can sea that the Peninsula is composed of seven countries: Vietnam, Cambodia, Laos, Thailand, Myanmar, Malaysia (partially) and Singapore. The island group consists of Indonesia, Malaysia (partially), Brunei and Timor-Leste. Before the time of decolonization, in the late 1940s, all of these countries except Thailand were colonies of European nations, namely the UK, the Netherlands, France and Portugal. They are now all independent nations, the latest one to access to sovereignty being Brunei in 1984 and Timor-Leste in 2002.