This is something I made for the Map of the Fortnight contest over on alternatehistory.com. The challenge was to make a map showing a scenario which is the reverse of a scenario from our timeline. The map shows a scenario in which France invades Germany via the Low Countries in the early 1900s, as a sort of reverse Schlieffen Plan.
The point of divergence from our own history is prior to the Franco-Prussian war - specifically France buys some of the breech-loading steel cannons that historically gave Prussia such a great advantage in the Franco-Prussian war from our timeline. It's not enough to win the war, but it's enough to prevent it going quite as disastrously as it did historically, so the Emperor isn't captured by the Prussian forces and the Second Empire continues on after the war.
The Franco-Prussian war was won before it was fought. Whilst France's soldiers sported the superior Chassepot rifle, this was the largest of only a few advantages the great power had. Prussia, meanwhile, had the advantage of the superior breech-loading steel artillery provided by Alfred Krupp, and whilst France had purchased some similar "Kruppstahl" cannons under the insistence of Emperor Napoleon III, most of France's artillery remained tried and tested muzzle-loading bronze designs. In addition to this, Prussia's use of railways to quickly mobilise and manoeuvre its forces had no equal in French military doctrine, and Prussia's army was the only one in the world with the benefit of a dedicated General Staff.
The Prussians beat France back from the border, and got so far as to besiege Paris. Though they never marched on the city themselves, their previous victories and the untenability of the French position led to a French surrender, and the transfer of Alsace-Lorraine - now Elsaß-Lothringen - to Germany.
France had been decisively defeated, but it was not out of the game for good. The French Emperor, Napoleon III, immediately set about analysing why France had lost, and how it could remove its disadvantages. A French General Staff was created, and French military doctrine was radically rewritten for a new age of warfare.
The Emperor was not only concerned with military doctrine and the nature of the army's equipment, but also with geopolitics. Traditionally France had one of the largest armies in Europe, but was unable to bring it to bear against any one foe due to its central position. In the run-up to the Franco-Prussian war, the Prussian Chancellor Otto von Bismarck had diplomatically isolated France, ensuring they had no allies to come to their defence. Though Napoleon III died in 1873, his campaign to break up von Bismarck's Dreikaiserbund - an alliance of the German, Austro-Hungarian, and Russian empires - continued with his son Napoleon IV, who helped break up the Bund in 1875, and obtained a mutual defence pact with Russia in 1879, in order to counter the dual alliance of Germany and Austria-Hungary. This broke France's isolation, and Napoleon IV instantly began diplomatic overtures with Italy and Britain, hoping to ensure the neutrality of both in the event of a war between France and Germany. These diplomatic ties improved greatly once Kaiser Wilhelm II dismissed Otto von Bismarck as German chancellor in 1890 - the Kaiser pursued a much more aggressive foreign policy than the Iron Chancellor von Bismarck had, alienating many nations who were leaning towards the German camp.
In the years between 1900 and 1910 tensions between the Franco-Russian bloc and the German-Austro-Hungarian bloc grew, as dissent in Austro-Hungarian Bosnia - supported by Kingdom of Serbia - threatened to bring Russia (Serbia's ally) and Austria to war. The French Chief of Staff at the time, Joseph Joffre, decided that France's current preparations for a potential war with Germany were inadequate, and so oversaw a radical change in French military strategy. Reasoning that German fortifications in Alsace-Lorraine were too formidable to attack from the front, Joffre formed a plan of attack whereby a number of French armies would march through neutral Belgium and the Netherlands, catching the German defences off-guard and surrounding them. This plan was further modified over the years, eventually becoming Plan XVII.
The opportunity for war came in 1911. Austria-Hungary claimed it had evidence linking a number of Serbian nationals to a series of terrorist attacks and assassinations in Bosnia, and demanded their extradition. Serbia refused to extradite a number of the alleged terrorists, leading Austria-Hungary to declare war, ostensibly to protect its territory from foreign terrorists.
Germany backed up Austria-Hungary's invasion, and said it would defend Austria-Hungary if any nation attacked it as a result. Russia, seeing itself as the protector of all Slavic nations and having provided Serbia with much aid in the past, declared war on Austria-Hungary and Germany, calling in its major ally, France.
Plan XVII was then put into action, with troops being mobilised and moved quickly into position for the initial assault. Unfortunately for France, it had underestimated many things - the effectiveness of the Russian army, the willingness of Britain to go to war over the issue of Belgian neutrality, the efficiency of Germany's own mobilisation plans, and the effectiveness of the Belgian and Dutch troops to name but a few. On paper the Plan looked perfect - almost infallible - but no plan survives contact with the enemy, and this is all the more true for plans as audacious and radical as Plan XVII.