The ideals that Europeans espoused at the outbreak of the First World War were very different from those which influenced Europeans during the...

This is a very complex question. The nineteenth century witnessed some of the most significant social, intellectual, and political changes in human history. For the most part, the most relevant changes to this question can be understood as part of what historian Eric Hobsbawm called the "dual revolution" of the early nineteenth century. The two events that initiated the "dual revolution" were the French Revolution and the Industrial Revolution, and they were important because they ushered in many of the changes that characterized the period. Let us look at a few of these changes.

First, the nineteenth century witnessed the rise of nationalism, which was itself kindled by the French Revolution and the Napoleonic Wars. Nationalist movements formed in virtually every region of Europe, most notably in Italy, where Giuseppe Mazzini's "Young Italy" movement advocated Italian nationhood as early as the 1820s. Several new European nations, including Belgium and Greece were formed in the two decades or so after the Napoleonic Wars, and Italy and Germany became unified through wars in 1861 and 1871, respectively. The emerging spirit of nationalism destabilized both the Austrian and the Ottoman Empires, a situation that continued well into the twentieth century, when it played a major role in sparking World War I. 

Another massive change was industrialization. Beginning in England, the Industrial Revolution spread to France and Germany in particular by the mid-nineteenth century. This created a large and disaffected working class in many European nations. It also contributed to national identity through the proliferation of "print capitalism," which tended to give one language priority over other dialects. Industry also helped power the mighty war machines that emerged in European nations, outfitting massive armies with modern weapons.

A side effect of the Industrial Revolution was the emergence of radical ideologies among the workers mentioned in the previous paragraph. These included socialism (both revolutionary and democratic) and anarchism. Many among the traditional aristocracy as well as wealthy industrialists and the comfortable middle classes feared the rise of these movements. Some governments tried to institute moderate reforms to lessen their appeal, while others opted for brutal crackdowns on radicals. 

These changes, I would argue, were the most significant in Europe during the nineteenth century.