my advice is dont worry terribly much about this... i'm almost positive this section is going to be entirely sources, you really only need the basics and as far as the reaction goes, it will be evident in the sources... if you're really desperate to find out about the reaction scratch around for sources published about that time from britain, germany, france... like in newspapers or posters and stuff.... that's what i'd do...
try that, it goes over the battle of the somme from all accounts and answers the question: was anything gained from the battle of the somme... hope that helps... do a little of your own research cause i only checked out google...