The United Kingdom possessed the largest navy in the world for the whole of the ironclad period. The Royal Navy was the second to adopt ironclad warships, and it applied them worldwide in their whole range of roles. In the age of sail, the British strategy for war depended on the Royal Navy mounting a blockade of the ports of the enemy. Because of the limited endurance of steamships, this was no longer possible, so the British at times considered the risk-laden plan of engaging an enemy fleet in harbor as soon as war broke out. To this end, the Royal Navy developed a series of 'coast-defence battleships', starting with the "Devastation" class. These 'breastwork monitors' were markedly different from the other high-seas ironclads of the period and were an important precursor of the modern battleship. As long-range monitors they could reach Bermuda unescorted, for example. However, they were still armed with only four heavy guns and were as vulnerable to mines and obstructions (and enemy monitors) as the original monitors of the Union Navy proved to be during the Civil War. The British prepared for an overwhelming mortar bombardment of Kronstadt by the close of the Crimean War, but never considered running the smoke-ridden, shallow-water gauntlet straight to St. Petersburg with ironclads. Likewise, monitors proved acutely unable to 'overwhelm' enemy fortifications single-handed during the American conflict, though their low-profile and heavy armor protection made them ideal for running gauntlets. Mines and obstructions, however, negated these advantages—a problem the British Admiralty frequently acknowledged but never countered throughout the period. The British never laid down enough "Devastation"-class 'battleships' to instantly overwhelm Cherbourg, Kronstadt or even New York City with gunfire. Although throughout the 1860s and 1870s the Royal Navy was still in many respects superior to its potential rivals, by the early 1880s widespread concern about the threat from France and Germany culminated in the Naval Defence Act, which promulgated the idea of a 'two-power standard', that Britain should possess as many ships as the next two navies combined. This standard provoked aggressive shipbuilding in the 1880s and 1890s.