Several lines of deep holes dug into the ground that faces another line of holes across a middle ground called No Man's Land. Barbed wire, mud, disease, machine gun fire, shells, etc used. They don't work because you can't have an effective war of attrition with 20th century production technology, so you just keep fighting and loosing soldiers.
Battle of the Somme - The Allies agreed to combine forces against the Central Powers. The United Kingdom came in with the most losses, with over 350,000 casualties. The British were still placing faith in their old war strategies which didn't work with new technology.