I am not an expert, but decisive and facile do not work in the two blanks because a war can be decisive, but in English, we do not usually say that war "is" decisive. The use of "is" in the sentence, "It is commonly believed that war is sometimes not only" implies a belief or an opinion, but that belief or opinion cannot be proven true or false until after the war has happened. Also, it may or may not be a "common belief" that war is decisive. We say a particular war "was" decisive because you can look at the past and assess whether a war was or was not decisive. We may also say that a war "may be" decisive because it is guessed or hoped that a war might be decisive, but there is no surety. You can also say a war "would be" decisive, but again this would be an opinion or someone's belief; it would not be a fact.
Here is a sentence where decisive is used to describe a war: {"The West's eventual triumph in the cold war was similarly decisive, because market capitalism prevailed even in closed societies" Purdum, 2011).} Notice the assessment about the war being decisive was made after it already happened.
Regarding 'facile': the word facile, according to Garner's Modern English Usage, "always [means] "easy" in one sense or another" but this word "may connote either proficiency or shallowness...Sometimes the word connotes the ease that comes with artistic mastery. More often, [facile] connotes triteness or oversimplification." Because moral choices are generally not considered shallow, trite or oversimplified, facile wouldn't work in the sentence you are asking about.
I hope this explanation helps.
