The following are extracts from Wikipedia
Hinduism in Southeast Asia has a profound impact on the region’s cultural development and its history. As the indic scripts were introduced from India, people of Southeast Asia entered the historical period by producing their earliest inscriptions around the 1st to 5th century CE.
Hindu civilization also transformed and shaped the social construct and statehood of Southeast Asian regional polity. Through the formation of Indianized kingdoms, small indigenous polities led by petty chieftain were transformed into major kingdoms and empires led by a maharaja with statecraft concept akin to those in India.
It gave birth to the former Champa civilisation in southern parts of Central Vietnam, Funan in Cambodia, the Khmer Empire in Indochina, Langkasuka Kingdom and Old Kedah in the Malay Peninsula, the Sriwijayan kingdom on Sumatra, the Medang kingdom, Singhasari and the Majapahit Empire based in Java, Bali, and parts of the Philippine archipelago.
The civilisation of India influenced the languages, scripts, written tradition, literatures, calendars, beliefs system and artistic aspects of these peoples and nations.
Expansion of Hinduism in Southeast Asia
Indian scholars wrote about the Dwipantara or Jawa Dwipa Hindu kingdom in Java and Sumatra around 200 BC. “Yawadvipa” is mentioned in India’s earliest epic, the Ramayana. Sugriva, the chief of Rama’s army dispatched his men to Yawadvipa, the island of Java, in search of Sita. It was hence referred to in Indian by the Sanskrit name “yāvaka dvīpa” (dvīpa = island). Southeast Asia was frequented by traders from eastern India, particularly Kalinga, as well as from the kingdoms of South India.
The Indianised Tarumanagara kingdom was established in West Java around 400s, produced among the earliest inscriptions in Indonesian history. There was a marked Buddhist influence starting about 425 in the region. Around the 6th century, Kalingga Indianized kingdom was established in norther coast of Central Java. The kingdom name was derived from Kalinga east coast of India.
These Southeast Asian seafaring peoples engaged in extensive trade with India and China. Which attracted the attention of the Mongols, Chinese and Japanese, as well as Islamic traders, who reached the Aceh area of Sumatra in the 12th century.
Some scholars have pointed out that the legends of Ikshvaku and Sumati may have their origin in the Southeast-Asian myth of the birth of humanity from a bitter gourd. The legend of Sumati, the wife of King Sagar, tells that she produced offspring with the aid of a bitter gourd.