In an attempt to restore the monarchy in France, the Roman emperor and the King of Prussia joined together. Their urge was to unite the European powers as they feared that similar unrest might evolve in their region. So they wanted to quash the revolution.
Apart from this, the Prussian King Leopard was the brother of Queen Antoinette so he had his interests to save the queen from the mob. So, Prussia and Austria pledged to intervene if the European Union agreed to quash the revolution.