This was basically the cocktail of the English empire. Tonic water contains quinine which is used to cure Malaria so the English would distribute units of tonic water to their soldiers stationed in India and Africa to keep them from getting sick.
As all things medicine related go, someone (likely troop members stationed in India) thought to add gin and lime juice to it to make it taste better. Thus the gin and tonic was born. This is the drink that for better or worse colonized the world.