Gin was an established medicinal tipple in the Netherlands as far back as the early 1600s. English soldiers fighting in Central Europe in the Thirty Years War brought "Dutch Courage" back with them and distillation in England began. 

Around the same time, the British were staking their claim in India, where malarial mosquitos were a bit of an annoyance. To make the very bitter anti-malarial quinine palatable, the Brits added a blend of sugar, lime, water and gin – and the original Gin and Tonic was born! 

Today’s tonic water contains less quinine than in the era of the British Raj, but the G&T remains the most classic gin cocktail.