Can an ideal be achieved?  If my understanding of what ideals are is correct (i.e., a mental conception regarded as a standard of perfection), then it seems that they are, by their very nature, unattainable (at least in a corporeal sense).  Yet, nations are built, wars are fought, and people are killed over ideals.  If they are only "perfect ideas", doesn't that seem a bit absurd and irrational?  
Is my understanding of what an "ideal" is incorrect?