What was the Hindenburg Line?

The Hindenburg Line was a vast system of defences in northeastern France during World War 1. It was constructed by theGermans.  This line was built along the salient of the German front.  A salient was a section of the trenches of front that was bulged out from the original line and created a large fighting area.  This line was a heavily armed and gaurded section of the front.  What would of happened if the Allied powers couldnt make it through the German Hindenburg line?