German Foreign Minister Joachim von Ribbentrop congratulates Yugoslav Prime Minister Dragiša Cvetković for joining the Axis alliance. Two days later, Cvetković’s government was overthrown in a coup d’état, which was then followed by an Axis invasion of Yugoslavia. (March 1941)(1300×821)

This sent Hitler into a fit of fury and he went on to swift punishment (with mass scale bombardment).
Interestingly, his diversion to crush Yugoslavia moved the schedule to attack the Soviet Union for a couple of weeks.
This proved to be fatal. Although German forces had great initial success and victory seemed within reach, winter eventually came, muddy terrain slowed movement and you know the rest.
Leave it to the serbs to change history again (see: Gavrilo Princip).