Dictionaries are available. Explaining is a factual thing. 'That rocket launcher can't go there because there is a big rock in the way'. Excusing is trying to make out that a choice was an inevitability when in fact there were other options. Putting a rocket launcher in a school is a choice, even if the area is densely populated. Making sure your rocket launcher is nowhere near your own citizens is an alternative, and most would think, preferable, choice.