Specifically asking about the battle of Verdun is confusing in your question considering the first part about Germany telling France they were going to attack. Germany and France had declared war on each other, so they both knew they were going to go to war with each other. This was in 1914. The most egregious thing Germany did in the eyes of the allies in the beginning of the war was to attack Belgium, a neutral country, in order that they might attack into France through a less defended area. The battle of Verdun did not take place until two years later, well into the war. Germany did not announce it was going to attack, as that is against all military common sense, and from what I remember the attack caught France off-guard, as both France and Britain had a tough year in 1915. Hope this is an ok place to start!

Check out some books like Hew Strachan's First World War