War and Necessity

This morning my four year old daughter was asking to watch a Christmas show on the TV.  I told her no, that we would wait until at least November before watching Christmas shows.  May is too early.  She asked what holiday is in May and I told her Memorial Day.  She wanted to know what that's about so I told her it is when we honor the soldiers who have fought in wars for us. 
She quickly told me that wars are terrible things and I agreed with her.  But, I told her, sometimes they are necessary.  Of course she asked 'why?'.  Well, that's a complicated subject, especially when talking to a four year old so I tried to boil it down to something simple.  I told her that sometimes people and countries do terrible things and sometimes the only way to stop them is by going to war.  I mentally prepared to figure out which terrible things to tell her about but fortunately she let it go at that.  I want her to be a little older at least before she finds out about things like the Holocaust or the Rape of Nanking. 
Machiavelli also wrote of the 'necessity of war' though I doubt he was as squeamish as I am.  He noted that the Romans:

foreseeing troubles, dealt with them at once, and, even to avoid a war, would not let them come to a head, for they knew that war is not to be avoided, but is only to be put off to the advantage of others

Roman ambition would involve wars.  The generals and Senate knew this.  More importantly, so did the citizens. 
A bit later, Machiavelli is writing about one of the King Louis' of France:

And if any one should say: "King Louis yielded the Romagna to Alexander and the kingdom to Spain to avoid war," I answer for the reasons give about that a blunder ought never to be perpetrated to avoid war, because it is not to be avoided, but is only to be deferred to your disadvantage.

I wonder about this.  Wars are caused by real disputes between states.  (I'm convinced of this and always annoyed when pseudo-psychology about 'boys and guns' is given as a war reason.)  How often are those disputes really settled peacefully?  If two countries claim the same land or resources, is war really inevitable?  And if it is, then should states hold back until there really is no other choice or should the simply wait until they have the advantage?  I know what the modern answer is, but is it true, or simply misplaced idealism?