Was it Warfare or Trade. I cannot find the answer to this question and im tired of looking
Buddhists follow three main traditions. There are those who adhere to the Theravada or Southern tradition, those who adhere to the Mahayana or Northern tradition and those who adhere to the Vajrayana or Tibetan tradition.
Long ago, Buddhism began to spread southwards from its place of origin in northern India to Sri Lanka, Burma, Thailand, Indo-China and other South East Asian countries. It also moved northwards into the Himalayan kingdoms (Sikkim, Bhutan, Nepal), Tibet, Mongolia and other parts of Central Asia, and also into China, Korea and Japan.
This was a fortunate development because Buddhism all but died out in India after the Moslem incursions of the 11th Century ce. In more modern times, the spread of Communism has also virtually obliterated Buddhism from various other countries where it was once strongly established (e.g. China, Vietnam, Tibet, etc.). There is now a resurgence of Buddhism in these countries. Nowadays, however, Buddhism is attracting an increasing following in Europe and the Americas. In Asia, it is thriving in countries like Sri Lanka, Burma, Thailand, Korea and Japan.
In the development of Buddhism after the Buddha’s death several schools and traditions arose. Most of them ceased to exist being absorbed by others and today we have three main schools of Buddhism namely the Theravada, the Mahayana and the Vajrayana.
Within each school there are different traditions.
Modern scholars tend to divide the schools according to the language used. Mahayana schools are therefore Sanskrit based schools, and Theravada Pali based.
After the Third Council, at the time of Emperor Asoka (the 3rd century B.C.E.), missions were sent to various parts of South Asia taking the teaching as finalized at the Council. It was this teaching, brought to Sri Lanka by Ven.Mahinda and Ven.Sanghamitta, which was written down in Pali about 25 B.C.E. as the Pali Canon.
Teachings of other traditions reached the South Asian countries but did not become established there. The school of Buddhism in the South Asian countries, such as Burma, Cambodia, Sri Lanka and Thailand, based on the Pali Canon is known as the Theravada school and sometimes as ‘Southern Buddhism’.
In the first century B.C.E. the language of the important schools of Buddhism in India was Sanskrit. About this time there were new developments within the existing schools. New interpretations of Buddhist ideas were developed in India in a religious environment, which included Brahminism and later Hinduism. These new developments went to form the Mahayana (Great vehicle) Teachings.
The special characteristics of the new Mahayana School were the emphasis on the Bodhisattva ideal, the aim of each person to become a Buddha and to work for the happiness and welfare of all beings. New Sutras (texts) were composed similar in form to the existing ones but expounding new ideas.
In China it was the Mahayana, which became established. Different traditions developed, each having its own monasteries and teaching facilities. Each tradition was based on one or more of the Mahayana Sutras. Similarly the Mahayana became established in Korea and Japan. In addition there were new traditions in Japan.
About the 7th century C.E. there were new developments from within the Mahayana. This was the growth of Buddhist Tantra activated by the mutual influence of Mahayana and Hinduism.
The aim of Vajrayana is the same as that in Mahayana, that is to attain Buddhahood, but the tantric practices showed a quick way to achieve this end. The different traditions of Buddhism in Tibet are as follows:
hough there are different schools and traditions, the fundamental parts of the Vinaya (the rules of discipline of monastic life) and the Dhamma go back to the Second Council (100 years after the Buddha’s passing away) before there was any division in the Sangha. This central part of the teaching is therefore common to all the schools’ traditions.
Different traditions give varying emphasis to different aspects of the teaching and practices. There are also additional teachings specific to particular traditions. The Abhidamma (higher systematic philosophy) has developed independently within the different schools and traditions, though even here we see many common features.
We need to remember the words in the Lotus Sutra,
‘There is only one yana – Buddhayana’, the path of the Buddha.
This is a very good question, I'm frm sri lanka and buddhism introdused to sri lanka by king ashoka in india, but its not by war or trade, King ashoka sent number of delegates to neighboring countries and kings of those countries accepted buddhism as their religion, however my country fought with enemies to protect the buddhism.
what do you mean "spread" its a religion not a disease. Anyway, go to the following links they should help you, if you care to read some.