The Neapolitan War, also known as the Austro-Neapolitan War, was a conflict between the Napoleonic Kingdom of Naples and the Austrian Empire. It started on 15 March 1815, when King Joachim Murat declared war on Austria, and ended on 20 May 1815, with the signing of the Treaty of Casalanza. The war occurred during the Hundred Days between Napoleon's return from exile and before he left Paris to be decisively defeated at the Battle of Waterloo. The war was triggered by a pro-Napoleon uprising in Naples and ended with a decisive Austrian victory at the Battle of Tolentino, after which Bourbon monarch Ferdinand IV was reinstated as King of Naples and Sicily. However, the intervention by Austria caused resentment in Italy, which further spurred on the drive towards Italian unification.

Before the French Revolutionary Wars the Kingdom of Naples was ruled by Bourbon King Ferdinand IV. Ferdinand was a natural opponent of Napoleon and was allied with the Third Coalition against him. However, after defeat at the Battle of Austerlitz and the Treaty of Pressburg, Ferdinand was forced to cede Naples to the French in early 1806.

Initially, Napoleon's brother, Joseph Bonaparte, ruled Naples. Then in 1808, Joseph was installed by Napoleon as King of Spain which led to the Peninsular War, and Napoleon installed his brother-in-law, Joachim Murat, as King of Naples.

Murat originally ruled Naples following the same legal and social system used in France, whilst still participating in Napoleon's campaigns. But following the disastrous Battle of Leipzig, Murat abandoned the Grande Armée to try to save his throne. As defeat in the War of the Sixth Coalition loomed, Murat increasingly moved away from a hapless Napoleon, eventually signing a treaty with Austria in January 1814 and joining the Allied side.

But as the Congress of Vienna progressed, Murat's position became less and less secure as there was growing support to restore Ferdinand to the throne. The most vocal of all Murat's opponents was the United Kingdom, which had never recognized Murat's claim to the throne and, moreover, had been guarding Ferdinand in Sicily, ensuring he retained the Sicilian throne.

When Murat was informed of Napoleon's plan to escape from exile in Elba on 1 March 1815, Murat sided with him once more and declared war on Austria as soon as he learned of Napoleon's return to France.

Joachim Murat declared war on Austria on 15 March 1815, five days before Napoleon's return to Paris and the beginning of his Hundred Days. The Austrians were prepared for war after their suspicions were raised when Murat applied for permission weeks earlier to move his troops through Austrian land in order to attack the south of France. Austria had reinforced her armies in Lombardy under the command of Count Heinrich von Bellegarde prior to war being declared.

At the start of the war, Murat reportedly had 82,000 men in his army, including 7,000 cavalry and 90 cannon, although this figure was grossly exaggerated to try to encourage Italians to join his cause. The real number was somewhere in the region of 50,000 men.