While the narrative that the kaiserreich had the "best" military probably predates Dan Carlin, his extended section in blueprint for Armageddon episode 1 about how this is the case has of course had those who take Mr. Carlin as the lone authority on historical matters popping up. The folks at /r/badhistory recently posted at length responding to someone who likely was influenced by Dan Carlin and rebukes the narrative that the Kaiserreich was all that great. This is not an attack on /u/elos_ who I think did a very good job with that post.
Some quotes I selected from that thread discussing the relative merits of the German Army include:
the German military was not the best trained. In fact, out of all the powers, military observers noted their distinct lack of adaptation to modern conflict.
They were also hardly the best equipped. The French 75mm flat firing artillery pieces fired 2-3x as fast as the German 77mm counterpart and, unlike the German heavy guns, did not need massive rail transport just to move them to the next battleground. Electronic communications was yet another area of massive neglect. While the French and British used it extensively, Schlieffen and Moltke were content to conduct maneuvers both in simulations and in the real deal by literally riding horses to hand deliver hand written detailed plans. In fact, messenger birds would make up the bulk of the German communication department, not modern radio or even telegraph.
These two quotes make the flaws in the German army seem very serious indeed, but I wonder if the pendulum has swung to far in the other direction.
But just from a purely snarky standpoint, yeah, the Germans were the best trained and best equipped and overall best army in the world. Guess that's why they won instantly in the Marne against those inferior French in 1914. Oh wait. Or why in 1916, with the cards on the table, they decisively crushed the French at Verdun and repelled the British counter attack at the Somme and then sued for a negotiated peace. Oh. Wait. Or why they were the first develop squad level infiltration tactics and small unit doctrine. Oh wait, that was the French in 1915 and the Russians in 1916. Or that's why they were the first to implement unit and platoon level tactical doctrine army-wide designed for the modern battlefield. Oh wait, that was the British in 1917.
So I guess my question is, how did they get to the Marne twice if the German Army wasn't so good, how did they last as long as they did in the face of these superior guns and innovations and tactics? I guess I'm just looking for people to set the record straight on what the German army exactly was. Was it a genius if slow to adapt superarmy whose martial prowess nearly crushed 3 great powers until the inevitable restrictions of geography and resources failed them, or are they just a mediocre army who took advantage of the conditions early in the war to make some progress before the might of the allies crushed them?
Because while they weren't some overwhelming force of "the best" military in existence, they were still damned good. Please don't take a snarky post I made on /r/badhistory in response to a steam review as some ultimate beacon of truth. It's very easy to swing hard in the other direction in something so off the cuff -- being more intent on disproving the other side rather than give a balanced view. So now that you have the mythology dispelled, it's time to get the balanced view of "Okay, they weren't terrible by any means" but in fact the Germans were a very respectable fighting force.
Here are two posts I recommend for context of Germany's conflict in the West and their fall:
The first post is a rather direct, to the point description of just all the battles throughout it all. Here's a non-AH post I made on the matter that is more readable/modern. I honestly prefer this one to the AH post above, as it's more concise, but it's not as rigorous so I'd look at both.
The short of it is that they held off in 1915 because 1915 was not the year of the West. There were battles, certainly, but as a whole in 1915 Germany threw the entirety of its weight Eastward, to deal with Russia, while the West was more or less a holding operation. The British meanwhile saw no need in bashing their heads against the trenches just yet -- they had a massive navy and intended to use it. So they went to the Ottoman Empire in hopes of causing their collapse, thus bringing Bulgaria and Romania into the war on the side of the Allies, thus causing Austria-Hungary to collapse and then Germany would be alone.
So we get to 1916 and these are cataclysmic battles about to happen. The Germans attack in Verdun and the French are battered terribly for months. The British counter attack at the Somme and batter the Germans terribly while the French fight back. So why didn't Germany collapse? Because, and I know it sounds cheeky, the Allies were not necessarily trying to. The Somme was designed to be a broad front operation that killed a bunch of Germans, not a breakthrough operation. The men were distributed evenly across the front for this express purpose on the first days. However, as you'll read above in my posts, after 1916 Germany was neutered. The Somme was the turning point and from then on it was truly them holding on rather than them dictating the terms in the West.
In early/mid 1917 breakthrough didn't occur because it was just a matter of how the war was fought. You would need to shell a region for days if not weeks before an offensive -- this obviously told them where you going -- and if you got any semblance of a breakthrough reserves would hit you from all sides and simply batter you back. This got even more pronounced when the Germans developed what's called 'defense in depth', detailed here, and the British countered with 'Set Piece' attacks, also detailed there. Meanwhile the French were just utterly destroyed by Verdun in the same way Germany was by the Somme, but much worse. Their manpower was stripped and the second they tried an offensive in April 1917 their men mutinied.
By the point hurricane barrages, only possible because of massive advances in spotting for artillery allowing them to sight much quicker and perform much more accurately, came into play along with Set Piece being in full swing and the British having all that conscript manpower available and trained and ready...they still didn't want to attack. Because the Americans were in the war, and they were coming. Why try some grand war ending offensive before millions of more men come to help? So they still bid their time and waited. You're seeing a recurring theme here. There is lots of waiting. In 1915 it was to see what happened in the East, for both parties, and in Summer-Winter 1917 and 1918 it was waiting for the Americans. In 1916, the year of cataclysmic battles and notably NOT waiting, the battles were not fought for decisive victory but for bleeding the Germans or French (respectively) as much as possible to prepare for said cataclysmic final decisive offensive. For the British it was hopefully a 3 pronged offensive from the Russians, French, and themselves in later years and for the Germans it was to destroy the French in such a way, and seize Verdun, such that they could bypass the mountainous regions of Eastern France and then swarm in a later offensive into Southern/Central France.
As for 1918, why that happened, and why it failed, see my posts above and you see, thus, the collapse of Germany.
I won't claim to offer a full answer to this, but one key factor was that their generals were far better (for the most part) than those of the Entente. The German military was essentially the old Prussian military with the best of the smaller German states such as Bavaria, Saxony, etc bolted on. They had been planning the attack on France since almost immediately after the Franco-Prussian war, correctly assuming the French would want that land back. In the west they were able to move quickly enough to seize the initiative and the strategically important ground time and again. British soldiers were noted complaining that the Germans always had the high ground and their own trenches were always on the low ground, so they'd often be knee-deep in muddy water and contract disease much more often. Even if their technology was a behind and their commanders weren't the best at adapting to circumstance, they still had that reputation. Perhaps an overblown one, but even a lie can scare the enemy.