Trench warfare is fighting between two opposite sides with soldiers dug into ditches on either side with "No Man's Land" inbetween. It usually consists of soldiers, sometimes tanks, and usually artillery. One side would send wave after wave into No Man's Land untill they gave up or the opposing Trench was overrun. Trench warfare was first used in World War1 and continued as a style of fighting for years to come.
Charles V: Men, You will be sent into Trench Warfare in France against the Germans.
Kaiser Wilhelm: We will also use Trench Warfare!
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.