On 23 July 1914, Austria-Hungary sent Serbia an ultimatum.
It was A-H's response to the murder of Franz ferdinand almost a month previously.
An ultimatum, of course, is where a country says: 'Inless you do a,b,c...' we will go to war.
A-H's ultimatum had ten demands - you can read them
here.
Most of them were things that A-H required Serbia to do to stop Serbian extremists and terrorists endangering A-H.
Serbia accepted NINE of the ten demands.
But it had a problem with No.6:
QUOTE
to institute a judicial inquiry against every participant in the conspiracy of the twenty-eighth of June who may be found in Serbian territory; the organs of the Imperial and Royal Government delegated for this purpose will take part in the proceedings held for this purpose
The Serbian government argued that this was against its constitution.
To allow A-H to take part in Serbian legal processes would in effect be a surrender to Serbian sovereignty - what's the point of having your own country if another country controls the laws there!
So Serbia rejected point 6 only, offering instead to take the matter of point 6 to an international court.
This wasn't enough for A-H, who then declared war on Serbia.
Some historians do not believe that Serbia was genuine in its response. They believe that accepting the nine and being so very reasonable was a trick to make A-H look unreasanable, and to get the suppor of public opinion on Serbia's side.