The Entente Cordiale in 1904. But really, it's because of Otto von Bismarck. Or really, his successor Leo von Caprivi's (mis)reading of balance of power, along with France and Britain needing to settle some colonial disputes, and France deciding that a united Germany was a bigger threat than Britain.

Some background. Back in the 19th century, the peace in Europe was ostensibly kept by the balance of power between the major powers. There are many reasons the contemporary interpretation of the post-Napoleonic peace is mistaken. Crimea being one of them, and also, the very many revolutions of the mid 1800s. But still, there hadn't been a cataclysmic war since Napoleon and lots of crowned heads thought it was because each of their states were evenly matched.

One pretty decent example of how this balancing worked comes from the summer of 1878. The Balkan states wanted to break away from the Ottoman Empire. They got Russian backing, so Serbia, Montenegro, and Greece got together and formed the Balkan League to declare war on the Ottoman Empire. This was ostensibly resolved by the Treaty of San Stefano wherein Russia got several buffer states and basically created Bulgaria.

Britain, Germany, and Austria-Hungary were not having this because Russian expansion to the detriment of the Ottomans was a bridge too far. The Russians bowed to pressure and decided to essentially re-do the treaty at the Congress of Berlin where Bismarck reigned the Russians in. Austria got Serbia and Bosnia-Herzegovina, and nothing bad ever happened again between Austria and Serbia things got interesting in 1914.

A lot of this sounds very much like Great Man History, but I've found it useful to see how people like Bismarck made decisions or felt constrained.

For example in 1866 during the Austro-Prussian war, Bismarck managed to engineer the unification of Germany while also marginalizing the other big German nation/empire, Austria. Prussia's victory ensured that the new German nation would largely be dominated by Prussia.

Bismarck had been trying this for some time. Before the united Germany, the German states had a customs union called the Zollverein. Think like the EU, but just for the 300+ German states that existed at the time. Bismarck had been trying to advocate for a more assertive Prussia, and not just via the Zollverein. Bismarck went as far as to suggest an alliance with Italy so as to have backup against Austria. War was not his first choice, but it worked eventually.

Even after fighting the Austrians, Bismarck made sure to align Austria, Russia, and Prussian-dominated Germany to isolate France. Bismarck even succeeded in getting Napoleon III removed by (allegedly) provoking a war, and seizing the coal producing part of France called Alsace-Lorraine in the Franco-Prussian war.

The ostensible reason was France's history of belligerence ever since the Revolution and the Napoleonic Wars. This means that Germany was effectively the power in central Europe, and Bismarck's careful balancing act was designed to hold the peace and the balance of power.

That said, Bismarck was just very good at quoting historical precedents to support decisions he made using other considerations. Prussia's domination of Germany wasn't about neutralizing France - it was just about Prussian domination of Germany.

But in 1890 Bismarck got his walking papers from the Kaiser. After that von Caprivi let the German-Russian treaty lapse. Then Russia signed a pact with France in 1894. Ten years later Britain and France signed the Entente Cordiale.

France entered these alliances for practical reasons. To shore up support against a strong Germany, but also to settle a couple of colonial disputes/issues it had with Britain. The fact that France, Russia, and Britain were allies wasn't lost to the Germans - I mean, the chief of staff of the German command came up with the Schlieffen Plan in the event they'd have to fight a two-front war against France and Russia. Who needs a an alliance treaty when we have the freaking Schlieffen Plan?

The reason was ultimately pragmatic. Germany took French land in the Franco-Prussian war. The German threat was more immediate. France fought and lost two out of three wars against a united Germany in 1870, 1914, and 1939. Without Britain (and Russia,and eventually the USA), France stood no real chance.

But this explanation mostly works if you buy into the balance of power argument. It's inherently a nationalistic one, and given the ethnic and religious diversity of Europe, I don't think it's actually right.

Consider that Austria only pushed Serbia around after the assassination of Franz Ferdinand because of Germany's “blank check.” Which the Kaiser only authorized because he didn't think that anyone would risk a cataclysmic war over Serbia. Not because the European powers were evenly matched, but because few imagined a perennial Balkan dispute would turn into a huge world war.

Yeah, balance of power literally makes no sense in many ways. Mostly because it's not clear to me that those who espoused it actually believed it.

But hey, if they actually believed it in 1904 then who am I to judge?