In the first 4 years of war, the Allied powers struggled to defeat Germany, but looking at the timelines, it looked like starting in the August of 1918, war tides changed swiftly and Germany soon after signs the armistice in November.
How did the Allies go from struggling for 4 years to quickly handing Germany their defeat in a matter of months?
They halted the German Spring Offensives, and then began to push them back since they now had sufficient forces at their disposal and logistical support (as well as a significantly weakened enemy) to do so. Combine this with the Blockade, and with the weak state of Germany's allies, Allied victory was not at all surprising, at least in hindsight. I have some other answers you may be interested in: