How did the aggressive nationalism push France and Germany to the brink of war in the early 1900s?

Answer 1

German aggression didn't start the First World War. Serbian Nationalists assassinated the Arch-Duke Ferdinand of the Austro-Hungarian Empire, and then revolted. Germany and the Austro-Hungarian Empire had a mutual defense treaty, necessitated by centuries of pressure on the Central European Powers by Russia and France (the Franco-Prussian War was not even cold yet). Austro-Hungaria had received assurances from the Russian Foreign Minister that Russia would not get involved on the side of the Serbs (Serbia is a Slavic nation), in exchange for which Austro-Hungaria would support it against Turkey, but when the Central Powers mobilized and began action against the Serbs, Russia reneged on its promise (the Foreign Minister was cashiered for making it) and declared war on Austro-Hungaria. Germany then declared war on Russia and France, having a treaty with Russia and wanted revenge for the Franco-Prussian War (which they lost miserably), declared war on Germany. Germany then attacked France, but had to march through the Low Countries (Belgium, Holland and Luxembourg) to execute its grand flanking strategy against the French. Britain was guarantor of the Low Countries', and warned both sides to respect the rules of war in them. But Belgian illegal combatants (according to the Geneva Convention you must wear a uniform to be a legal combatant) began sniping and killing German soldiers. The Germans then broke the rules by executing civilians in retaliation, so the British declared war on Germany.

The notion that the Germans were aggressors in the First World War was pure Propaganda. The real culprit was the 'Grand Diplomacy' of the era, and the belief at the time that war was a legitimate method to achieve diplomatic goals. Its a mistake to lay the blame on Germany, and unjust. Doing that was what caused the Second World War.

Answer 2

France and Germany were at odds over Alsace-Lorraine. The French wanted it back because they had lost it in the French-Prussian war. The aggression ended up turning on Germany.