The Second Italian War of Independence, also called the Franco-Austrian War, the Austro-Sardinian War or Italian War of 1859, was fought by the Second French Empire and the Savoyard Kingdom of Sardinia against the Austrian Empire in 1859 and played a crucial part in the process of Italian Unification.
A year prior to the war, in the Plombières Agreement, France agreed to support Sardinia's efforts to expel Austria from Italy in return for territorial compensation in the form of the Duchy of Savoy and the County of Nice. The two states signed a military alliance in January 1859. Sardinia mobilised its army on 9 March 1859, and Austria mobilized on 9 April.
On 23 April, Austria delivered an ultimatum to Sardinia demanding its demobilisation. Upon Sardinia's refusal, the war began on 26 April. Austria invaded Sardinia three days later, and France declared war on Austria on 3 May.
The Austrian invasion was stopped by the arrival of French troops in Piedmont that had begun in late April.