"Over the Top"
"Over the top" was an expression soldiers used in WWI when referring to attacking soldiers rising out of their own trenches to assault the enemy. In order to attack the enemy, soldiers had to climb over the parapet of their trenches and cross "no mans land."
This picture shows soldiers climbing and crossing the parapet (layers of sandbags) in order to attack the enemy. This picture is a great example of how soldiers had to climb many layers of sandbags just to take a few shots toward the enemy's trench. These soldiers can be referred to as going "over the top."