A few reasons why the Hundred Days Offensive was so successful:

1. The Kaiserschlacht offensives had massively overstretched the Germans. Ludendorff had aimed to use reinforcements from the now defunct Eastern Front to smash through the Allied defences and end the war before the Americans could arrive in large enough numbers to turn the tide of the war. Instead what the Kaiserschlacht achieved was giving the Germans control of large swathes of tactically unimportant land in exchange for their last reserves and the deaths of many of their best soldiers who had been grouped into the stormtrooper brigades and suffered disproportionate losses. Furthermore as Operation Michael et al were smashing through the Allied positions, the Germans came across Allied supply dumps and started looting and getting drunk, seeing how well the British and French soldiers were still living compared to conditions on the German side and how even after all the efforts of the Kaiserschlacht the war continued, German morale started to suffer serious degradation. This combined with the almost disastrous conditions of the German Home Front where the British blockade was destroying the Home Front and people were coming close to starvation. Unrest and political upheaval grew with every month and although the German Army would fight until the very end it was suffering from serious morale problems by the end as commented upon by British soldiers when capturing Germans during this period.
2. Allied tactical improvement. The operations planned during the Hundred Days were so much more sophisticated than the blundering at the Somme or the Nivelle Offensive. The BEF (British Expeditionary Force) had been on a ‘learning curve’ since ’14 adapting to the modern state of war and increasing their tactical ability with every battle fought and every lesson learned. Although battles such as the Somme had been a disaster they had taught the British important lessons and even by early ’17 you can see a clear improvement in British tactical planning. By ’18 the Allied armies (especially the British) had absorbed these lessons and developed the beginnings of true combined arms tactics. Infantry tactics had become devolved to the men on the ground and focused around the platoon rather than large formations of men. This allowed the infantry to be much more efficient and reactive compared to the botched large formation basic orders of the Somme for instance. Each infantry platoon was also far more heavily equipped than their equivalent in ’14-’17 with more machine guns and grenades than ever before. Behind the lines the artillery had mastered its art, was able to fire bombardments without being able to see the enemy to maximise surprise (using maths to calculate their position), was now extensively using hurricane bombardments to further maximise the surprise and was heavily using creeping barrages and leaping barrages. It had become so effective at co-operating with the infantry that a barrage could timed to perfectly match the advance of the infantry, covering their attack on the German trenches and giving the Germans no time to react until the advancing infantry were upon them before then lifting forwards to the German rear lines to prevent the Germans from reinforcing their positions under attack. Elsewhere tanks were present in ever increasing numbers supporting the infantry and enabling them to break through trenchworks while providing mobile heavy fire. It’s important to note these were not the speedy panzer divisions of ’39-’45 and still performed a support role to the infantry. Finally aircraft were being used in increasing numbers, doing innovative things such as air-dropping supplies, dropping smoke bombs over the battlefield and chasing and strafing retreating German soldiers preventing them from regrouping once a breach had been made in the German lines. At the same time Whippet Tanks, which were more mobile versions of the main British tank (although still far from what we’d consider mobile) were also involved in widening and exploiting any breaches made. All these factors combined: improved infantry tactics, massively improved artillery tactics and increased and innovative usage of tanks and aircraft combined to give the Allies almost the precursor of modern warfare and something that the Germans had no reply to. They could now easily break into German lines and make advances of 5/6 miles a day in some cases but this would be useless if it wasn’t tied to improved strategic decision making.
3. At the same time strategic decision making improved massively. Rather than simply batter the same positions for months at a time even though the chance for breakthrough had ended, as soon as the first few days of an assault were over and the breakthrough slowed down the Allies switched their point of attack. While this was happening all the Allied armies were attacking at the same time, the British from Flanders and Northern France, the French in the centre, the Americans in the south in the Meuse-Argonne Offensive. This constant stream of hammerblows across the entire front and the constant switching of the focus prevented the Germans from reinforcing their positions and no sooner had one breakthrough been halted, another would open up somewhere else on the front. This stretched the Germans so much that by late October it had broken them into a full on fighting retreat. Behind this huge exertion of resources and manpower was the sheer materiel superiority of the Allies, to enable such a concerted and continuous chain of offensives required a huge amount of resources and by this point of the war the Allies (now with America on full war footing) were massively outproducing the Germans in almost every metric possible.

So the reasons why the Hundred Days was so effective was the declining state of the German Army and it’s failure to win the war with the Kaiserschlacht, the sheer materiel superiority of the Allies and finally their ability to put together the lessons of the past four years into true battle winning tactics that look more like the tactics of ’39-’45 than they do the tactics of ’16 and before. America’s entry into the war wasn’t so much the ‘turning point’ in terms of their military contribution, that was still relatively speaking lesser than the British and French even in November ’18 but more in terms of forcing the Germans to launch the Kaiserschlacht in early 1918 and gamble on victory before the Americans could arrive in large numbers. Unfortunately that gamble failed for them and weakened the German Army, then when the Allied launched the Hundred Days, the culmination of four years of bloody lessons, there was only going to be one result.