When did the Germans realize that they could not win World War 1?

I think there is a paradox here. The German front line troops were undefeated in 1918. But there was great hardship in Germany itself. Economically Germany was bankrupt. Although Russia was defeated, Austria/Hungary was also in poor shape. Soon the Americans would bring forces to bear which would breach the German lines. Military defeat would be obvious as 1918 became 1919: It was simply a matter of time.