Back to the original post, I doubt that anything short of the assasination of a sitting US President - by itself - could be used by the US as the raison d'etre for a formal war or conquest against the perpetrating nation. Important to note that, while the preempted attempt against Bush I in Kuwait may well have put Iraq in Dubbya's sights from day one (and may have been his reason for the war), this was never used publically as justification for the US Invasion. As noted this happened while Clinton was president and was used to justify punitive bombing of Iraq, but not an invasion.
Also, emotionally, I suspect that killing a Crown Prince/Heir to the Throne carried far more emotional baggage to Austrians in 1914, than the assassination of any US Vice-President would today. Franz Ferdinand was the automatic sucessor to the King. When you kill the Crown Prince you are killing future Emperior. An American Vice President is merely an elected official that becomes president if a President can't finish his term. This is nothing like being a designated heir to a king.