Archduke Franz Ferdinand of Austria became heir to the Austrian throne after his cousin committed suicide in 1889. In 1895 he met Sophie Chotek, a lady in waiting (and, incidentally, a descendant of one of those defenestrated in Prague). They kept their relationship a secret for two years, as Sophie was not eligible to marry into the royal family. However, in 1900 they were allowed to marry, but none of their descendants could succeed to the throne. Sophie could not appear with him in public, taking other means of transportation and sitting in a different area during events.
There was a loophole: When Franz was acting in military duty, his wife could share his rank and they could appear together in public. In June 1914, they were in Sarajevo en route to a town hall reception when their motorcade was bombed in an assassination attempt. Franz and Sophie were uninjured, but 20 others were not so lucky. After the reception, they decided to visit the wounded in the hospital, but their motorcade took a wrong turn. In the confusion, one of the assassins, who happened to be grabbing a sandwich nearby after the earlier failed assassination attempt, saw the motorcade and approached the car, shooting and killing both Franz and Sophie.
And thus began World War I. Austria-Hungary (of the Central Powers) declared war on Serbia (of the Triple Entente Powers), the allied countries on each side declared war on each other, and the rest of that story is better known than the catalyst.
An interesting note: the man who threw the first bomb took a cyanide pill and jumped into the nearby river. However, the cyanide was old, so all it did was make him throw up, and the river he jumped into was only four inches deep. Bad planning on his part – he was severely beaten by the crowd before being taken into custody.