The proximate cause of the invasion was Ferdinand's double-crossing of Napoleon. Wanting to keep things quiet in southern Italy, Napoleon and Ferdinand signed a convention that specified that the French would evacuate Apulia. In return, the Kingdom of Naples would stay neutral in the impending War of the Third Coalition. No sooner had the French occupying force marched away than Ferdinand admitted British and Russian armies into his kingdom. In December 1805, Napoleon's armies crushed the armies of Austria and Russia. When the Russian force in Naples was recalled, the British expedition also withdrew, leaving Ferdinand's kingdom exposed to French retribution.