The modern submarine changed the equation of naval warfare during the Great War, and navies had developed few tactics and weapons to counter them. In the opening months of the war the U-boats of the imperial German navy demonstrated that they were a real threat to the Royal Navy. On August 9, the SM U-15 encountered the cruiser HMS Birmingham, which used the most reliable method at the time to destroy a submarine. The tactic of ramming U-boats during the great war deserves to be remembered.