Why does Shakespeare chose to open Much Ado about Nothing with the background of war and its aftermath besides metaphorical reasons?
Two reasons for opening Much Ado about Nothing with the background of war have to do with social situation and character development. In the first, choosing a setting that includes the background and aftermath of war is an elegant way to collect a lot of unrelated men together in one place at the same time. Further, these men, having just braved the hardships of war are ripe for the comforting benefits of marriage. This device aids the development of the story and plot that Shakespeare wants to tell.
Having the aftermath as part of the setting allows for deep character development of several characters that occupies a short period of time as the opening scene proves. In the conversations between Leonato, Hero, Beatrice and the Messenger, much information is stated and inferred about a good number of important characters. For instance, Claudio's character is developed in reference to his actions in war and to his Uncle's feelings toward him.
In another instance and most famously, Beatrice's character is developed through inference from her dialogue and her attitutde toward Benedick is elaborated upon, while, at the same time, his character is developed. So much information on so many characters would take much longer to develop if a less elegant means were used.