Brinker's father is a foil for the boys, a way for Knowles to bring out the difference in attitude toward the war between the generations.
Mr. Hadley is a veteran of World War I. However, enough time has gone by since he was in any fighting position to allow him to forget the bad aspects and romanticize the whole image of marching off as a brave young man going to serve his country.
Your war memories will be with your forever, you'll be asked about them thousands of times after the war is over...if you can say that you were up front where there was some real shooting going on, then that will mean a whole lot to you in years to come.
Brinker and Gene, as the ones facing the immediate involvement and the inherent dangers of entering the battle, do not sympathize with Mr. Hadley's viewpoint. The boys try to distract his criticisms with examples of how war technology and procedures have changed since he was in the military, without success. They struggle to remain respectful in deference to his age and experience, but don't yield to his arguments or attempts to persuade them regarding their approach to enlistment and the war.
It's all that World War I malarkey that gets me...I'm not any kind of hero, and neither are you. And neither is the old man, and he never was, and I don't care what he says he almost did at Chateau-Thierry.