The two opposing forces in WW1 were a) Allies and Central Powers.
The First World War started as a two-sided war between Austria-Hungary and Serbia in 1914. It evolved into a war in which thirty two countries took part. Britain, Russia, France, the United States and Italy formed The Allies. These countries battled against Austria-Hungary, Bulgaria, Ottoman Empire and Germany, known as the Central Powers.
In The Red Badge of Courage, when Henry wakes up in camp the day after fighting, he first think he is in a Charnel place because the sleeping men around him look dead.