The Dutch responded to the growing intimidation by enlisting large numbers of armed merchantmen into their navy. The English, trying to revive an ancient right they perceived they had to be recognised as the 'lords of the seas', demanded that other ships strike their flags in salute to their ships, even in foreign ports. On 29 May 1652, Lieutenant-Admiral Maarten Tromp refused to show the respectful haste expected in lowering his flag to salute an encountered English fleet. This resulted in a skirmish, the Battle of Dover, after which the Commonwealth declared war on 10 July.