The First World War was very recent history at this point in the novel. All the colonies and dominions of the British Empire, like Canada, were automatically drawn into the conflict when Britain declared war on Germany in 1914. 67,000 Canadian soldiers lost their lives.
An M.C., or Military Cross, is a military decoration that was created in 1914. In the period referred to here, it was awarded to commissioned and warrant officers in the British Army, for acts of exemplary gallantry. It could also, at that time, be awarded to officers of British Empire dominions.
A.D.C. stands for aide-de-camp, an officer who acts as an assistant and secretary to a high-ranking member of the military, such as a general.