World War I was as inevitable as the explosion of a wagon of nitroglycerin speeding along a bumpy road. The war was the explosion; its catalyst was an intricate net of European alliances, nationalistic rivalries, and the death throes and inevitable collapse of an archaic and decadent system of imperial dynasties.
The tripwire, of course, was the assassination of Austria-Hungary's Archduke Ferdinand. It seems ridiculous today that an incident like that could result in general war in Europe; however, the fact that it did is indicative of how ready and willing Europe was to start killing a whole generation of their young men.
The alliances that forced the major powers of Europe to line up on each side of a minor Balkan dispute were what dragged France to support Russia and Germany to back Austria-Hungary. Soon we had the spectacle of everyone declaring war on everyone else. Germany's march through Belgium, a catalyst that brought Great Britain into the fray, was another indication that whatever the excuse, Kaiser Wilhelm had imperial designs on his neighbors. It is likely that if the assassination could have been prevented, Germany would have found another excuse, probably in some colonial territory in Africa.
The Germans might have been able to forestall the entry of the United States into the European land war. However, German unrestricted submarine warfare, resulting in American deaths at sea, and the disclosure that German diplomats had promised Mexico some of the U.S. southwest in return for Mexican support so angered American public opinion that the deal was closed.
Could the Germans have avoided these blunders? Probably, but such would have been out of character and not consistent with what the Kaiser felt was in German interests. The tremendous arms and naval buildup in Germany during the years before the war, as well as German resentment over being left out of the race for colonies, rushed the Kaiser and his military establishment headlong into war.
Much of Europe, at the beginning of the 20th century, was a system of bloated plutocracies and festering popular resentment. Russia, for example, entered the war against Germany, and the Czar's soldiers threw down their weapons as the Bolshevik revolution spread and ultimately took Russia out of the war and relieved eastern pressure on Germany and probably extended the war as German redirected its efforts towards its western front. Colonial rivalries, a naval and heavy weapons arms race, and the foolish treaties that drew everyone into a four-year conflagration were the inevitable beginning to the end of an era.
Yes, World War I was inevitable. Tragically, so was World War II. As the vengeful, short-sighted policies of England and France imposed crippling penalties and humiliation on Germany, yet another generation of young men grew up to populate row on row of white cemetery crosses.