Southeast Asia lies between the Indian and Pacific Oceans and includes countries like Vietnam, Thailand, Indonesia and the Philippines. It has a tropical climate and terrain that made travel and communication difficult historically. Kingdoms arose in Southeast Asia that were influenced by Indian and Chinese religions, culture and political systems through missionaries and trade. The Khmer Empire dominated mainland Southeast Asia for centuries and built elaborate irrigation systems and cities, most notably Angkor Wat in Cambodia. Island kingdoms like Srivijaya ruled extensive empires through controlling trade routes and taxing commerce. Vietnam developed a unique culture while absorbing influences from China during over 1,000 years of occupation.