I read an analysis of hundreds of assassinations. In not one case did the assassin achieve his political goals. In this case, had the duke come to power, he most likely would have granted the political objectives the assassin’s wanted. He had written and said that the current policy was outdated and a bit of autonomy was called for.
It wasn’t his assassination that caused the war. Austria was looking for a reason to attack. It would have found one eventually. The German Kaiser had imprudently given Austria a blanket guarantee of support, not realizing what it would mean. Then, the Kaiser’s number two urged Austria to strike while the Kaiser was on holiday on his yacht and (in those days) out of the command loop.
In my opinion, World War One is still the most destructive war in history. WW II caused more damage, but WW One was the one that destroyed Christianity in Europe (which led to the inevitable brutality that happens when man loses his fear of God), that set up new borders that we're still fussing about today, that introduced new weapons that were so devastating that Armies still prepare for them (for example, a huge bulk of my time training when I was in Army boot camp and the National Guard was against Gas attack).
In World War One, Jews in Poland and Russia welcomed the Germans because they liberated them from Russian Pogroms, the Japanese were our allies and got great kudos for their excellent treatment of Prisoners Of War (many German POW's voluntarily stayed in Japan after the war to build new lives).
America's involvement in WW One was devastating to our psyche. While we only had 100,000 or so men killed in action, the bulk of it was in a 3 month period. We thought we were "making the world safe for democracy"- instead, we were just a pawn of the western European powers as they cynically got payback. This led to a strong anti-military feeling in America that lingered until Hitler invaded Russia (which caused Democrats and American Leftists to become gung ho pro-war, and the Jap Sneak Attack on Pearl Harbor (which pi$$ed off the rest of the nation).
Even then, while The Greatest Generation was quite patriotic, you never see letters or stories about "love of country" or "fighting for a cause" like you did during the American Revolution or the Civil War. This was because of the cynicism many WW I vets had about what they went through. This bled to their children, and their thoughts were more about getting the job done and going home, not letting down their buddies, etc. Ironically, all the bad things the Germans were accused of in WW I, they actually DID in WW II (American troops in WW II didn't believe the news about Germans slaughtering Jews and others, they thought it was just propaganda).
Actually, that's exactly backwards.
Franz Ferdinand was a proponent of modifying the Dual Monarchy to a Triple Monarchy, giving the national status under the Habsburgs already held by the Germans and Magyars to the South Slavs. This would presumably have eliminated the South Slav discontents, as it had for the Hungarians. Promptly squashing the ambitions of the conspirators to build a Greater Serbia incorporating all South Slavs.
They didn't care about making life better for the South Slavs, particularly under the Habsburgs. They wanted to break up the Habsburg Empire so the South Slavs could be added to Serbia.
IOW, they specifically wanted FF dead because he was a competitor for the allegiance of South Slavs, not just because he was a Habsburg.
If Oswald was, as reported, a rabid Marxist whose goal was to push the United States leftward, it may have been decades in the making, but by making LBJ POTUS, he certainly gave things a big shove in that direction. If the gunman/men were somebody other than Oswald, we have no way of knowing if their political goals were realized as we have yet to positively ID the trigger puller. If, as I personally suspect, LBJ was the puppet master, his goal was most definitely realized.