During World War I, the Germans used their U-boats or submarines to terrorize military and merchant ships. By 1917, Britain was losing merchant ships at an alarming rate, especially considering they were importing much of their food supply. Fearful of being starved into submission, Britain developed the convoy system to protect their merchant ships from U-boats. Convoys are essentially merchant ships surrounded and protected by ships of the Royal Navy. Although convoys mitigated the U-boats impact, it was the U-boat that emerged victorious with 5,000 Allied merchant ships sunk compared to 178 U-boats destroyed during the war.