The Horizon (The Royal Marines Saga, 3) by Douglas Reeman

World War I, 1915, Jonathan Blackwood fights from the sea, supported by the Royal Navy in the battlefields of the Dardanelles and Gallipoli, watching the slaughter mounting around him, helpless to save either himself or his men. The days of the scarlet-coated marines of his forefathers are gone, giving way to a new warfare of grim trenches and ruthlessly efficient machine-guns.