The short answer is amazing politicians and diplomats with foresight second only to those who can see the future.

The long answer begins with the Congress of Vienna. It was where the peace deals were made following the French Revolutionary and Napoleonic Wars. After decades of war and almost total domination of Europe by Napoleon the were utterly against 2 ideas: any one power of Europe getting too powerful; and liberal revolutionaries violently overthrowing the old dynasties.

On the second point they pretty much all agreed that none of them wanted their feet amputated at the neck and so they all agreed to lend support in the event of liberal revolutions/rebellions.

The first point was far more complicated. After the turmoil caused Napoleon there was no way any of the major powers were ever going to let anyone else get too powerful. You may heard the term “The Balance of Power”. This is it in all it’s glory. They divvied up the land of Europe so that the victors got their spoils of war but so no one got in a position where they had significant advantages over their neighbours. And even if that were to happen in the future (it did, I’ll get to that) they signed many alliances so that if anyone went to war it would include always include at least one of the great powers. This meant it was very tricky to try and take out any smaller neighbour you might have because you risked having to take on one or even two of the most powerful countries in the world.

During this time there were also many, many great leaders and negotiators that were able to keep the peace and the balance of power. A few examples: Alexander III of Russia, Otto von Bismarck of Prussia (later Germany) and von Metternich of Austria are but a few.

This was all disrupted by the unifications of Italy and (more importantly) Germany. But the skilful negotiations of Bismarck kept Germany from going to war for many years even after his death. It was the failures of people like Nicholas II of Russia and Wilhelm II of Germany that led to The Great War.