Another good strategy is to get your DoW target to first declare on everyone else, by bribing them if necessary (preferably luxuries/strategics/GPT which will be canceled when you DoW):If you feel the compelling need to DoW another civ, you may want to Denounce them first. This seems especially true if your relationship with them has them at Neutral, Guarded or Hostile. It more or less tells the other civs that you aren't being a warmonger, but that the other guy really does deserve a kick in the fanny.
- If you have them DoW city states, it's easy and free influence with them once you kill some of your target's units. Also guarantees that they won't ally with those city states later on when you fight (as long as it doesn't drag on too long).
- If you have them DoW other civs, you got potential allies/distractions, you may be able to divert their troop placements, and the diplomatic consequences are far less severe (other civs may be angry with that AI and you will get "Share a Common Foe" benefits).