 Britain forced peasants in its colony, India, to grow opium poppies so it could sell the opium in China, although the importation and sale of opium were illegal under Chinese law.When a Chinese government official seized British opium cargo in 1841, Britain declared what became known as the first Opium War.Britain won the war and was in a position to impose on the vanquished Chinese a one-sided treaty with onerous treaty obligations.