In my opinion, the reasons for the Entente victory in the First World War are primarily the following reasons, all on the German side:
1. Poor diplomacy. Antagonizing both Britain and Russia, who had been enemies for a long time? Failing to get even Italy and Romania on your side or at least neutral? Come on. Looking back, it's like German diplomats were trying to lose the war.
2. Poor war planning. So, (speaking from a German POV here) we have a short border in the west that can be easily defended, flanked by a neutral power (Belgium) which, if invaded by France, would ensure British neutrality. On the other hand, we have a long Russian border which is at its closest point some 200 miles from Berlin, and a relatively weak ally we can't trust to guide its own border. So, who do we choose to concentrate on? (Hint: it's not the obvious one). So, of course we outflank the French by a maneouvre through Belgium bringing Britain into the war and increasing anti-German sentiment worldwide, and bet all our chances on a risky plan that even the slightest delay could shred to pieces. Then, if that isn't enough, we make some silly changes to the plan while we're executing it, to ensure our own defeat.
3. All right, so despite the silliness of the Schlieffen plan and the risks it exposed the German and Austrian eastern border to, Germany survives and manages to defend a long frontline in France and Belgium while simultaneously pushing around the Russians when they're bored. Germany is actually doing fairly well for itself militarily - with the exception of the Italian front, the war has been carried into enemy territory everywhere. So subsequently, the Germans decide on unlimited submarine warfare... out of desperation? Had they delayed it a few months, Russia's collapse into revolution would have rendered this risk that pushed the USA over the edge and into the war unnecessary.
4. So, finally the Russians have been beaten and the Bolsheviks have been convinced to sign a peace favourable to Germany. Despite the Germans making so many stupid choices, their military advantage has still brought them to the situation where Russia is knocked out of the war and they can focus on the western front. Then, rather than supporting Austria to follow up Caporetto (which might have knocked Italy out of the war and thus significantly lowered Franco-British morale, as well as freeing up lots of troops) or having some clear targets in mind for the offensive in France (like the Channel ports) and focusing all their attention on that, the Germans went for some silly offensives that gained quite some ground but failed to yield a strategic advantage. A lot of human errors were made, but given the amount Germany had made already, it's ridiculous. And then, yeah, an Entente victory had become inevitable after Germany had exhausted itself and the American forces were coming in in much greater numbers. America (and Britain, and France) saved the day, but only because the Germans gave them the opportunity.