Let me know how it is. Tannenberg is a highly underappreciated battle. The Germans won, of course, but in the meantime they old Prussian Elite had become nervous due to the Russian hordes invading their homeland. That nervousness probably ended up costing Germany their best (and probably only) chance to win WWI. Short version:
When the Prussian Elites raised alarm over the Russian invasion of East Prussia the issue became such that the Imperial General Staff had to respond. Their response was to transfer an entire army from the Western Front to the Eastern Front to meet the threat.
Unfortunately for the Germans, this transferred Army was in transit (W->E) when the decisive actions against the Russians occurred in East Prussia and again in transit (E->W) when the Battle of the Marne stopped the German advance into France on the Marne river.
Notably, the German army that was withdrawn from the Western Front due to the apparent threat in the East was taken from the exact spot where the French discovered and attacked a large gap in the German line. Had it not been for the Russian invasion of East Prussia these troops would never have been taken off of the Western Front and it is entirely possible that France would have been knocked out of WWI. Given that Tsarist Russia subsequently collapsed it is entirely possible that this could have dramatically altered the outcome of WWI.
Barbara Tuchman's Guns of August is a great read about the diplomatic attempts to avoid the war and the early days of WWI.
WWI is fascinating to me, moreso than WWII. The technological changes during WWI were, IMHO, even more immense than during WWII. Look at aircraft, for example. In the fall of 1914 when the war started the aircraft mostly looked pretty much like the Wright Brother's original flying machine. They were mostly wood and cloth bi-planes with almost no armor or armament. By the fall of 1918 there were some very modern-looking all metal monoplanes with substantial armor and armament. Similarly, on the ground at the beginning of the war Cavalry was a major weapon and rifles were similar to the ones used by the armies of Napoleon and his enemies a century earlier. By the end of the war there were tanks and machine guns.
The cultural changes brought on by WWI were also immense. At the beginning troops in Austria Hungry answered to an Emperor, troops in Russia answered to a Tsar, troops in Germany answered to a Kaiser. The families of the Emperor, Tsar, and Kaiser had ruled at least parts of their respective lands for centuries. When the dust settled after WWI the Tsar and his family were all dead and Russia was ruled by a Politburo. The Austro-Hungarian Emperor died during the war (he was over 80 when it started) and at the end of the war the empire was dissolved into many pieces and his successor lived out his post-war life in Madeira in exile. The Kaiser fled Germany at the end of the war and lived out his days as an exile in the Netherlands.
WW I rifles were all fast loading with cartridges fashioned usually after the Mauser, and they were rifles. The Napoleonic era weapons were muskets, unrifled, and slow loading in the main. The British had some Rifle Companies but their rifles were even slower to load. The bolt action was a significant advance in rapidity of fire as was the cartridge.
Other advances were of course in rifled artillery where the French 75 was a breakthrough type weapon and the Germans built monsters like the "Paris Gun", use of railroads (also seen in the US Civil War), entrenchments in the battlefield seldom used in the Napoleonic Wars, and of course poison gas. And of course naval ships were considerably advanced over 1814.
The Germans at that time were undisputed masters in the field of chemistry. They had invested the Haber process which meant their explosives were no longer dependent on mining of saltpeter etc. And they devised use of agents like chlorine and mustard "gas" and phosgene. The nerve agents did not come along until the mid-1930s from insecticide research, also led by the Germans (and not used in WW 2 by the Germans).
The northern belt of France was pretty much demolished in WW One, including the Champagne region, which led to the strategy in 1940 of fighting the next war defensively and in Belgium. That didn't work out so well.
One of the interesting things about WWI was the extent to which the military establishments of the European powers had failed to learn lessons from the American Civil War. The ACW demonstrated how the advent of rifles (with their much-longer range than muskets) expanded the killing zone out in front of defending infantry. This--combined with defenders improving their positions with trenches and/or abatis made it much more difficult for frontal attacks to succeed. No longer could smooth-bore artillery move close enough to defending infantry to use their anti-personnel ammunition, Napoleon-style--the horses drawing the guns were too easy for rifle-armed defenders to hit.
And rifles themselves improved. The standard ones were essentially rebarrelled muskets, with reduced bore. They were muzzle-loaders, just like Napoleonic-era muskets, with paper cartridges that soldiers had to tear open with their teeth, pour the powder down the bore, then push the patch and the Minie ball down with a ramrod. By the end of the war there were many units equipped with breech-loading rifles and carbines, and even some repeating, metallic-cartridge rifles.
So defending armies went to ground, entrenched, and put up barricades, and attacking armies usually lost. Cavalry moved to the margins of the battlefield, being used for reconnaissance and security missions, and occasionally in economy-of-force roles, such as Buford's division on Day 1 of Gettysburg, but virtually never in frontal charges except against other cavalry (Custer's fight against elements of Stuart's command east of Union lines at Gettysburg on Day 3 is a good example) or disorganized, scattered, or retreating infantry.
The bayonet became less important. There was some prevalent humor that the only bayonet wounds delivered were in the backs of soldiers who were already breaking. Bayonets didn't become useless, but because the killing zone of defensive positions had so increased in depth, bayonet charges seldom reached their objectives.
All of this was there for the European armies to learn, but they mostly didn't. They wrote off the lessons of the ACW as the product of mass, poorly trained armies fighting under the command of amateur generals. They had to experience the horrible offensives of 1915 before they learned the truth.
