The Sidecar was purportedly created in Paris during World War I by an American Army captain who liked to travel to his drinking establishment in the sidecar of his motorcycle. But there are other conflicting accounts of the origin of this drink. There are references to a London bartender named Pat MacGarry as the inventor of the drink.
There is also murkiness on the proportions of the ingredients of a Sidecar. One ratio is equal parts of the three ingredients and is known as "the French school". There is also an "English school" that has a ratio of 2 parts Cognac, 1 part Cointreau, and 1 part lemon juice. This produces a drier drink where the Cognac has more of a chance to peek out from under the lemon juice and sweetener.
David Embury in The Fine Art of Mixing Drinks had a different ratio: 8 parts Cognac, 2 parts lemon juice, 1 part Cointreau. Maybe the Cointreau was sweeter back in David's day? Otherwise that is a tart drink, though with the large amount of Cognac relative to the other ingredients maybe you can get away with it. Somebody try this ratio and let us know in the comments.
I am an adherent of the English school with the following proviso: equal parts of Cointreau and lemon juice produce a very tart drink. So I drop the lemon juice down somewhat. A sugared rim on the glass appeared in the 1930s and it complements the English school Sidecar's dryness and strength quite nicely.
This really is a great drink. It is basically a brandy-based Daiquiri or Margarita (stripped down to its base ingredients). Do try to use good Cognac or good French brandy. Spanish brandy will make a softer drink. And by all means, don't substitute Triple Sec for the Cointreau.