The British Prime Minister, the Duke of Newcastle, was optimistic that the new series of alliances could prevent war from breaking out in Europe. However, a large French force was assembled at Toulon, and the French opened the campaign against the British by an attack on Minorca in the Mediterranean. A British attempt at relief was foiled at the Battle of Minorca, and the island was captured on 28 June (for which Admiral Byng was court-martialed and executed). War between Britain and France had been formally declared on 18 May nearly two years after fighting had broken out in the Ohio Country.