The greatest tragedy of the war is that Franz Joseph signed off on it without much intel besides the promises the warhawks like Hotzendorf made about the war being a quick matter and nothing to be concerned about.
It's honestly kind of shocking how terrible events unfolded in 1914 between July and August with all the inept and apathetic diplomats and politicians and world leaders. I know most people don't think nuclear armageddon actually will happen, but the thought of repeating the July Crisis is appalling to me, especially because the start of WW1 is always dumbed down to, "Franz Ferdinand gets shot in Bosnia by a (Bosniak) Serb nationalist and all of Europe goes to war."
There are two lessons of the war, one lower-order and one higher-order lesson, at least in my view.
The lower-order lesson is that sometimes war
is inevitable and you cannot hope to stop it. Think Hitler in Poland or Hotzendorf with the world; sometimes no matter what you do the opposing interlocutor is just so cynical and so in favor of war, even in spite of all credulous reasons not to, that nothing you can do will prevent it. You can only make it less painful by being so lopsidedly prepared that their attack is immediately blunted. This is an obvious lesson and doesn't really help in viewing the history of the beginning of the war or it's expansion.
The higher-order lesson, which actually has been learned, is that the civilian leadership must always be in charge of the military and be kept abreast of the tools and capabilities of their militaries. Diplomats need to know that, for instance, if your military has to go to war that it will always involve invading two neutral neighbors in order to defeat a third, and that that is intended to make defeating a forth neighbor easier. In almost every European country the opinion of the civilians was one of shock that the military could not just suddenly change timetables, or half-way through a mobilization change course, or that the plan for war
required half your troops to already be in the process of conquering a neighbor in order to ensure enough space for troops to be unloaded from congested rail yards. This is best exemplified by Kaiser Wilhelm's response to von Moltke advising him the trains were going and could not be stopped even if he wanted to - "your father would have given me a better answer". People realised the huge mistake that was occurring as the armies began to mobilize, and yet nothing could be done to stop it one the button was pressed. Likewise, the militaries were shocked at the civilian leadership's handling of events; they basically had the attitude that someone else had made their bed, and God be damned, they will ensure they are not the ones to sleep in it. Their warplans envisioned basically only total wars with invasions of several neighbors; in Germany's case they invaded France and Belgium, solidifying their informal alliance, and guaranteeing France's entry when it was not clear whether their treaty with Russia even applied - pre-war, the "alliance" was very much stiffling to both. France did not consider the Balkans an essential interest for Russia and therefore not for France, and Russia did not consider Africa an essential interest for France and therefore not for Russia, so they both constrained each other during crises to prevent war.
So you essentially had a civilian leadership that knew little of the capabilities or plans of their own armies, then demanded a solution to a problem they created. The military in response gave them one option, and that option was essentially a poison pill for Russia (involving Germany in a pointless Austro-Serb conflict) and Germany (expanding a large war into a Great War by invading France and Belgium out of fear and France would be involved). In the case of Germany, the fear was so great that no matter what the crisis was in the pre-war period, whether it involved France or otherwise, every outcome would have resulted in 3/4ths of the German Army wheeling through Belgium.
Applied to today, I think we are far safer. Not only are great powers constantly intwined through various means, whether that's the red telephone in the White House and Kremlin or the WTO or whatever else, civilian leadership is firmly in control of the military of all great powers, even in Russia and China, and is intimiately aware of their purpose, capabilities, means, plans, etc. War is still possible, but our ability to control events even at speed has far surpassed what we could in 1914.