It is also worth remembering that everybody thought the war in 1914 would be short, there were very few people who thought it would drag out for years. All involved probably did some mental math based on actions like the the Franco-Prussian war and figured that the same level of casualties, or even twice as many, was worth it.
If they would have known, which we do with hindsight, that it was going to take 4+ years of fighting it may have never happened.