On September 2, 1870, two of Europe's most powerful men met and made an agreement that would change the course of European history and set the stage for World War 1.

One of them had just suffered a humiliating defeat from which he would never recover. For the other, the meeting was the culmination of years of patient, cunning political manoeuvring to unite his country, making it the most dominant in Europe.

The first man was Napoleon III, nephew of Napoleon Bonaparte and Emperor of France; the second was the brilliant statesman Chancellor Otto von Bismarck, effectively the Prime Minister of Prussia, and engineer of the war that had just caused Napoleon's downfall.

Napoleon III (left) meets with Chancellor Otto von Bismarck (right) at Sedan on September 2nd, 1870

THE ROAD TO WAR

It's ironic that a war between France and what would become Germany, was sparked by a question of Spanish succession.

In 1868, Queen Isabella II of Spain had been ejected from the throne and, following an interlude with no monarch, was to be replaced two years later.

Wilhelm I of Prussia - at that point, the largest and most powerful state in a region that would soon become Germany - stood to benefit from a succession that went to a German royal.

He'd nominated Leopold of Hohenzollern-Sigmaringen, a Prussian prince.

But France, fearing encirclement, had a thing or two to say about that.

Leopold (left) and Wilhelm I (right) might have encircled Napoleon III (centre) and France

France had suffered no end of political upheaval since the revolution of 1789, aspiring to republicanism but descending into terror and chaos, then transitioning back into imperialism and monarchy under Napoleon.

A republic was restored following the defeat of Napoleon in 1815, but his nephew, Louis Napoleon Bonaparte, had been elected as a popular President of the Republic from 1848 to 1852.

Unfortunately, he proved not to be the best republican, staging a coup in 1851 when the French Parliament and constitutional rules blocked him from serving a second term as President.

Emperor Napoleon III, as he became, was still on the throne in 1870 when the question of Spanish succession came up.

He instructed the King of Prussia, through his diplomat Count Vincente Benedetti, that Leopold was not to ascend to the Spanish throne.

For his part, Wilhelm I, who had a cordial relationship with Benetti, did not press matters too firmly and was more than willing to back down.

But Napoleon insisted that the Prussian king also guarantee that Leopold never accept the Spanish throne, and that he, Wilhelm, apologise to France for having tried to influence Spanish succession in the first place.

Wilhelm I (centre) in Bad Ems, where he received French diplomat Vincente Benedetti

Wilhelm politely declined to do this, and, since things had now been wrapped up, also ended further official contact with Benedetti before seeing him onto his train home.

He dispatched an account of this polite meeting to Bismarck who, in what today might be described as spin, immediately published his own version of what had happened in the newspapers:

"His Majesty the King thereupon refused to receive the ambassador again and through his adjutant informed the ambassador that he had nothing further to say".

The story had the effect he’d planned, landing in Paris newspapers on Bastille Day, inflaming public opinion there against Prussia over this ‘insult’, and likewise angering many a Berliner who'd read it in their own newspapers.

France subsequently declared war on Prussia - Bismarck had sprung a trap that was intended to bring Prussia's southern neighbours into a larger German fold, with Prussia at the helm.

GERMAN UNIFICATION

Germany was not officially a country until 1871.

Up until that point, it had always been a region, much as 'Europe' is today.

The area that now comprises Germany, Austria, and northern Italy was known as the Holy Roman Empire in the early Middle Ages (and was known as the First Reich to the Germans).

The Austrians came to have more authority and influence over the 300+ individual states that existed in the region, but when this began to wane in the 1700s, the area was soon contested by Prussia and France. The latter forced Austria back as Napoleon took over more and more territory, officially ending the Holy Roman Empire in 1806.

In the aftermath of the Napoleonic wars, these 300 German states were reduced to 39.

Member states of the German Empire in peach, with Prussia in blue

Following these changes, Prussia, under Wilhelm I and his Chancellor and Prime Minister Otto von Bismarck, embarked on a series of wars with territorial rivals Denmark, Austria, and then France in 1870.

War was brought about with France by a trap that was set (if, indeed, that was Bismarck's intention, which is still debated by historians) at the end of the war with Austria in 1866.

By this point, Prussia and her allies had formed The Northern German Federation, an arrangement that dominated, and no doubt intimidated, southern German neighbours such as Bavaria.

They were soon convinced to sign a secret military alliance for 'mutual protection' in the future.

It was this alliance that was activated in 1870.

FIGHTING BREAKS OUT

Following the furore over the way the Ems Dispatch was communicated to the press by Bismarck, France - and the alliance of German states led by Prussia - lumbered into war.

Both sides wrestled with problems of logistics: how to move vast armies by rail, then by horse and foot; how to supply food and fodder; and how to maintain command and control over what would be the largest continental armies to take to the field that century.

The result was a messy learning period, but it was the Germans who proved to be more organised, having dedicated the years prior to the war to expanding and professionalising their army staff - precisely so that it could grapple with these kinds of problems.