The success of the military revolution at Naples seriously alarmed the powers of the Holy Alliance, who feared that it might spread to other Italian states and so lead to a general European conflagration. The Troppau Protocol of 1820 was signed by Austria, Prussia and Russia, although an invitation to Ferdinand to attend the adjourned Congress of Laibach (1821) was issued at which he failed to distinguish himself. He had twice sworn to maintain the new constitution but was hardly out of Naples before he repudiated his oaths and, in letters addressed to all the sovereigns of Europe, declared his acts to have been null and void. Metternich had no difficulty in persuading the king to allow an Austrian army to march into Naples "to restore order".