Hostilities reach a climax in Europe upon Germany invading Luxembourg days after Archduke Ferdinand is assassinated. The same day, Germany and Russia declare war against each other, and France mobilizes.
At the end of July, Europe’s tenuous peace collapsed when Austria-Hungary declared war on Serbia. By 5 August, Russia, France, Belgium, Great Britain and Serbia had all lined up against Austria-Hungary and Germany. One month later, the bloody Battle of the Marne was fought outside Paris, halting Germany’s advance to the sea and effectively deciding the future course of the war.
World War I would be a terrible war of attrition, won by whichever side was willing to commit the greater number of men and resources to the lethal trenches.