If not for the US joining, there was a very real chance of them winning the war. Everybody was running low on supplies, manpower, and morale at that time. Their mistake was moving through Belgium, which brought the UK into the fight. The idea was that it was the fastest way to take Paris and knock France out. That way they wouldn't have to fight a war on two fronts. Makes sense as a calculated risk, but nobody was expecting trench war and the stalemate in movement it caused, turning the war into a battle of attrition. If Britain remained out, the Germans could have had an open water way to the north (supposedly) to import supplies needed to keep going longer and it would have been over for France and Russia.