pffff they havent even begun to taste the horrifying aspect of trench warfare...
barricades and cover fire are only the beginning aspects traps, carpet bombing, pits, explosions, ambushes, long range sniping, pillboxes and anything you can imagine... and when you finaly weathered all of it you captured but one trench
sabaton made a very fitting song about the trench warfare in the first worldwar "price of a mile" and that was what happened when both sides had the same tactical knowledge...
those invaders will be slaughtered a few trenches in