In fact, according to Tanqueray's global brand ambassador, Angus Winchester, during the Thirty Years' War, England's army witnessed Dutch soldiers fortifying for battle by drinking genever. Genever is the traditional liquor of the Netherlands and Belgium. It is from this that gin evolved.
In order to be considered gin, a liquor must contain juniper. However, each brand contains it's own group of flavoring aromatics—anything from dried citrus peels to licorice. Genever is not a type of gin—it is generally barrel-aged and has a malty taste more similar to whiskey than gin.
Seamus Harris, world traveler and cocktail blogger metaphorically distills it down to this: "While the urbane and clean-shaven London Dry is the dominant gin style today, Genever is its extravagantly whiskered, baccy chewin', gold prospecting grandpa." I'll drink to that.
