In which decade did World War I begin?

The 1910s - it began in the summer of 1914, five weeks after a Bosnian Serb shot the heir to the Austro-Hungarian throne in Sarajevo. The gap between the assassination and the outbreak is the part usually compressed: nothing was inevitable in those five weeks, and a great many people had time to make decisions that they made badly.

What turned a Balkan crisis into a continental war was a set of alliances that had been built to prevent one. Each was defensive, each obliged its members to mobilise if a partner was attacked, and mobilisation timetables were so rigid - millions of men moved by railway on schedules planned years ahead - that generals told their governments the plans could not be halted or reversed once started. The machinery designed as a deterrent became the reason nobody could stop.

It killed something like 17 million people, ended four empires, and produced the settlement out of which the next war came. It also killed the assumption it started with: that a European war would be short. Almost every plan on every side assumed a decision within months, and the whole of the twentieth century's subsequent politics is downstream of that being wrong.