Serbia agreed to almost every point of the AH ultimatum regarding the investigation of FF’s murder. The one they rejected was about Serbian police working under AH supervision or something like that. Something no nation would accept, both extremely humiliating and dangerous.

Anyway, rejecting an ultimatum doesn’t give you right to invade. It may be polite and civilized to give ultimatums in general, as compared to outright invasion without warning, but see the previous sentence.