(darn! Don‘t you just hate it when you press "Enter", and then think of something else you wish you‘d said ... ?)
You may already have heard of this link, but just in case you haven‘t...
My younger brother introduced me to "Ask Jeeves" - a really useful, general purpose Internet search/reference site: http://www.askjeeves.com/
(I did a search on "trench warfare", and a gazillion hits came up)
Also, it dawned on me you might be able to use a famous quote as part of your introduction (I routinely abuse and overuse quotes, and haven‘t flunked a history course yet ...), so here goes:
"My God, did we really send men to fight in that?"
(Lieutenant General Sir Launcelot Kiggell: On seeing the terrain and the mud after the battle of Passchendaele, 1917)
"War sank in to the lowest depths of beastliness and degeneration ... For years the armies had to eat, drink, sleep amidst their own putrefactions. Bit by bit the old campaigner‘s memories and young solder‘s dreams were engulfed in machinery and mud."
(Sir Ian Hamilton: The Soul and Body of an Army, vii, 1921)