Austria was engaged in the 1716–1718 Austro-Turkish War, while France and the Netherlands needed peace to rebuild their shattered economies. Attempts to resolve the situation through diplomacy failed and in June 1718, a British naval force arrived in the Western Mediterranean as a preventive measure. Emboldened by their success in Sardinia, in July 1718 the Spanish landed 30,000 men on Sicily but the strategic position had now changed. Austria signed the July 1718 Treaty of Passarowitz with the Ottoman Empire, and on 2 August, joined Britain, France, and the Dutch in the Quadruple Alliance, which gave its name to the war that followed. The Treaty of London, the defence pact that established the Quadruple Alliance, was written in Latin and signed on 2 August 1718 (Old Style: 22 July 1718). War Outbreak The Spanish took Palermo on 7 July, then divided their army; on 18 July, Marquess of Lede opened the siege of Messina, while the duke of Montemar occupied the rest of the island. On 11 August, a British squadron commanded by Sir George Byng eliminated the Spanish fleet at the Battle of Cape Passaro. This was followed in the autumn by the landing of a small Austrian army, assembled in Naples by the Austrian Viceroy Count Wirich Philipp von Daun, near Messina to lift the siege by the Spanish forces. The Austrians were defeated in the First Battle of Milazzo on 15 October, and only held a small bridgehead around Milazzo. In 1718, Cardinal Alberoni began plotting to replace the Duc d'Orléans, regent to the 5-year-old King Louis XV of France, with Philip V. This plot became known as the Cellamare conspiracy. After the plot was discovered, Alberoni was expelled from France, which declared war on Spain. By 17 December 1718, the French, British, and Austrians had all officially entered the war against Spain. The Dutch would join them later, in August 1719. San Sebastián The Duc d'Orléans ordered a French army under the Duke of Berwick to invade the western Basque districts of Spain in April 1719, still under the shock of Philip V's military intervention against them. Berwick successfully besieged San Sebastián and also entered northern Catalonia. In both regions there was support for the invaders from leading local figures, some of whom lobbied for them to be permanently annexed by France. Spain attempted to counter this by launching its own expedition to Brittany, in the