What was Austria's ultimatum to Serbia ? (WWI)

An ultimatum is a "do this, or else" sort of a statement
that one country makes to another.  In this case, Austria-Hungary was telling Serbia
that Austria would go to war unless Serbia did a number of things.  Austria purposely
made the terms of the ultimatum very harsh because they wanted it rejected -- they
wanted war.

Among the things they demanded from Serbia
were:

• Serbia's government should renounce all
  propaganda against Austria -- propaganda about Slavic
  nationalism.


• Serbia had to arrest any Serbian officials
  involved in the assassination of Archduke Franz
  Ferdinand.


• Serbia had to allow Austrians to come into
  Serbia and conduct the investigation -- to allow Austrians to come in and decide who
  should be arrested in connection with the
  assassination.

This last clause was the one
that was most unacceptable because it is very unusual for a country (Country A) to let a
hostile country (Country B) come in and conduct police operations inside Country A. 
It's like letting Country B take control of Country A and that's not
acceptable.