Flanking

Flanking is an optional rule (DMG251). 'Back attacks' and the like have been a part of the core rules since the early 1980's at least and we added this optional rule when we converted to 5th Edition to stay faithful to how we had played in the past. We are no longer playing with this optional rule for the following reasons:

• Flanking causes all kinds of strange (and in real-life what would be suicidal) movement in combat, notably the 'running around an enemy after you were in melee range with it' trick to set up or take advantage of flanking opportunity. In 3.5 and earlier, such movement would provoke one or more AoO's and thus was highly discouraged, but with the stingy AoO triggering rules that are core to 5e, this is no longer the case. This should put the person doing such a thing at a great tactical disadvantage, but there is no longer a real penalty for doing this, so keeping flanking in the game seems to be all-benefit with few drawbacks
• This kind of square-level micromanagement takes a lot of time to plan between party members and a lot of time to think about from the DM's side so the monsters do not give up obvious free flanks nor fail to take advantage of them when the opportunity prevents itself. Removing flanking across the board simplifies combats on both sides of the table