The ancient and noble art of stomping across someone else's cabbage patch and becoming deeply unpopular in the process. Throughout history there have been many invasions of sorts, but they usually end in tears and are not always successful.
Like a fine whisky, the ingredients must be just right for an invasion to succeed and the invader should go to great lengths to ensure that the invadee doesn't kick his arrse. Also to be taken in to account is the aftermath and if possible or desirable, some sort of exit strategy.
A superb example of an invasion is D Day - the Allies went to enormous trouble to ensure the Germans got a shoeing. A bad example of an invasion is Iraq, where the spams decided that strategies other than military ones were unnecessary. Militarily text book, politically? Cake and Arse Party.