I wanted to give this some thought before replying.
I don't think that the assassination of Franz Ferdinand was - or had to be - the necessary, sufficient or inevitable trigger for war, although that turned out to be the case.
Actually, I have walked that very section of that street along the quays in Sarajevo, a section where you come to a nice bridge - which leads to the New Town - to the right, and a short, surprisingly sharp junction leading down to the Old Town to the left, - the quays themselves continue on straight ahead; this was the street where the Archduke's motorcar took a wrong turn from the quays into the Old Town on June 28, 1914, and came to a complete halt on a short incline, while the driver fumbled for the reverse gear, in order to reverse back out and up onto the quays, where the rest of the entourage waited.
And, of course, this was also where Gavrilo Princip, then 18 years old and tubercular, turned down for terrorist activities by Serb military intelligence on the grounds of ill-health, one of the few conspirators from that morning's botched assassination attempt who hadn't yet been rounded up, (a bomb had been hurled earlier at the Archduke's car, he had seen it coming and deflected it with his arm on to the bonnet of the following car where it exploded), who was sitting, drinking coffee, and feeling very sorry for himself, when the Archduke's splendid motorcar came to complete a halt just in front of his disbelieving and astounded eyes.
As he hadn't been rounded up yet, he still had his revolver; he leapt on the running board of the stationary car, and poured the contents of his revolver into the Archduke Franz Ferdinand and his morganatic wife, the Archduchess Sophie, - who gamely tried to protect one another, theirs was a love marriage - killing them both and setting into motion the sequence of events that led to Europe being at war within two months.
Nevertheless, I don't think that the assassination of Franz Ferdinand made war inevitable; certain choices could have been made even at that time in high summer which would have defused matters.
But I do think that by 1914 an unstable situation where constellations of alliances, extraordinarily inept political leadership, nationalist passions, and idiotic military planning which meant mad adherences to train timetables, had arisen which, in turn, almost guaranteed that some sort of trigger to set off armed conflict was increasingly likely to occur.
Several potential clashes - any one of which could have given rise to war - had been averted or defused in the years immediately prior to the war.
But some "issues" or problems existed that the political imaginations of 1914 were unable to address, and some of these problems long postdated the Congress of Vienna, sorry
@yaxomoxay. Thus, the problems the Congress of Vienna attempted to address in 1815 were not the problems of 1914.
The Congress of Vienna was an attempt to return to the status quo ante prior to 1789, and restore 'conservative' hegemony across Europe, reimagine a France in a relatively non-threatening stance - politically or ideologically or militarily - way to the rest of the continent, a France simply restored to bog-standard Great Power status, ruled by a king restored to power.
Much of the rest of the 19th century was about dealing with - and making more palatable to the regimes and societies of Europe - the explosive effects (political, social, economic, cultural) of the twin revolutions of the late 18th century, the Industrial Revolution (economic) and the French (and American) revolutions (political).
In essence, that meant slowly coming around to the idea and reality of enabling and accepting and facilitating middle class participation in politics, society, culture and public life (and in some cases, by the end of the century, extending the franchise and the right of political participation to the working class and peasantry as well). That, in turn, gave rise to other issues (class based politics, nationalism etc) but none of this would have needed a continent wide conflict (that had happened in 1848) to address the issues generated by such changes.
The real issues of "high politics" that made 1914 different from 1815, were firstly, the decline of the Ottoman Empire (which was the first of two enormous changes that brought into question the raison d'être of the Habsburg Empire - if the Turks no longer threaten Europe, do we need the Habsburgs to 'protect' Europe?) and secondly, the rise of Germany, which brought into sharp relief the identity issue of what it meant to be German - a very different form of what it meant to be German from that multinational version represented by the Habsburgs - and also displaced the Habsburg Empire as the key political, military (and cultural) force in central Europe.
Moreover, the rise of Germany threatened France in a way that the Habsburg Empire had never done, (not least re matters such as Alsace Lorraine). So, the template of 1815 (and indeed, 1878) no longer sufficed.
