I mean, it fought agains Britain(biggest commercial/sea empire?), France(traditionally biggest land power), Russa(largest empire in size), that is already 3 very tough enemies that had colonies all over the world, not counting other smaller nations that also contributed. Germany may have had the best army in that moment, but its allies(AH, Ottomans, Bulgaria..) were not even near as good as Britain, France and Russia, and they were isolated from trade with most of the world by sea blocade. So how did they get so close to victory, what was their economy and trade based on?
What I'm going to respond with comes from a myriad of sources, but mainly from the Guns of August.
For starters, Russia was woefully unprepared for war. They had precious little wire for secret communication, lots of internal divisions, and their mobilization process was inefficient at best. This allowed the Germans to commit relatively few resources to the eastern front, especially because all of Austria-Hungary's forces were focused against the Russians.
On the Western Front, the Germans had the advantage of striking the first blow, and fighting against a French high-command that was solely committed to the offensive, believing that elan (revolutionary morale) would overrun the German invaders. AS I'm sure you're aware, the military technology made offensives extremely costly.
Having gained the initiative and favorable position on both fronts, the static nature of contemporary warfare lent itself to a drawn out war, as immediate victory was almost impossible to achieve due to the ease of defensive positions. This, coupled with Germany's occupation of much of France's iron deposits allowed them to endure for three years. The Russian Revolution in 1917 allowed them to focus all of their resources on the Western Front, further elongating the war.