World War I was shaped by the new vehicles developed during the four years of conflict. A century after the start of the war, we’re looking back at the most remarkable vehicles—the planes, cars, tanks, ships, and zeppelins—it helped bring about.

For British merchant vessels operating during World War I, few things were so terrifying as the submarine. The German navy used the Unterseeboot, or U-boat, to sink 5,000 ships measuring more than 13 million gross register tons during the war.

As the war geared up, the Germans and the British believed the big battles would be fought with huge ships like the HMS Dreadnought and its sisters. Submarines, they figured, would be limited to coastal defense, preventing blockades by enemy ships and serving as lookouts.

Instead, the war saw the first widespread use of the technology. The earliest wartime vessels were rudimentary, according to historians at the Western Front Association. Most used diesel engines (Germany started the war with some gasoline-burning U-boats, but soon embraced diesel) and had a range of more than 4,000 miles, but they couldn't go very fast or stay under very long. German subs had the edge over those of the Allies when it came to reliability and armaments—some early models even had anti-aircraft guns—but the torpedoes of the era were accurate only to 3,200 feet or so.

The Germans, with a navy significantly smaller than that of the UK, kicked off the modern era of submarine warfare by sending U-boats into the North Sea to improve its odds at sea. The vessels quickly proved their might, sinking HMS Pathfinder and HMS Formidable, among others.

The Brits, worried about losing hugely expensive battleships to U-boats, kept their big guns near home. They formed an effective blockade of German ports using smaller vessels, stopping vital supplies from reaching shore. Desperate to strike back, Germany turned its U-boats on the unarmed civilian merchant ships keeping Britain fed and armed with supplies provided by the United States.

U Boat 139 which sank the Lusitania shown in an undated photo. Location unknown. The British liner was sank on May 7, 1915, by a German submarine off the southern coast of Ireland. About 1,150 men, women and children, perished, 114 of which were Americans. Among those Americans who perished were Charles Frohman, theatric manager; Alfred G. Vanderbilt and Elbert Hubbard, author. Sinking aroused great indignation in U.S. (AP Photo) AP

That shift in strategy signified a significant shift in modern warfare. Noncombatants have always died in war, says Dr. Jan S. Breemer, a submarine warfare expert and professor of national security affairs at the US Naval War College in Monterey, California. But Germany made a deliberate choice to to sink civilian vessels without warning. This would be echoed in World War II, when there was less concern for the risk of collateral damage during wholesale bombing of European cities.

The U-boat provided tremendous stealth. They would remain submerged until finding a target. Then it would surface, advise the crew to abandon ship, then bring it down with deck guns or send a boarding party to do the job with explosives. Torpedoes were expensive, and so used only when absolutely necessary. When Britain began arming merchant ships, the Germans began firing without warning.

By 1915, the North Sea was declared a war zone and U-boats adopted a policy of unrestricted submarine warfare. Germany publicly declared its submarines would destroy all enemy merchant vessels in the waters around Great Britain. There were, in the eyes of Germany, no neutral vessels. Hundreds of merchant ships were destroyed, including the RMS Lusitania in May 1915, with 128 U.S. citizens aboard. The sinking of the Lusitania helped garner public support for America to join the side of the Allies, which it did two years later.

The Allies were at a loss for how to respond to the U-boat threat. "The submarine was the absolute weapon of World War I," Breemer says. They were difficult to locate and hard to attack. "Navies didn't have any traditional responses for dealing with the submarine.” Between 1914 and 1918, the allies deployed nearly 10,000 ships, thousands of planes, and more than 100,000 mines to combat the U-boat threat, which totaled some 340 submarines. The Germans lost 178 U-boats during the war but sunk 5,000 ships.

The Allies eventually won the war on land, but the success of the U-boat campaign underscored how important, and devastating, submarine warfare could be. Once a submarine moves beyond a known location—it's home port, for example, or a known choke point like a channel or a strait where sonar can be used effectively—it is all but impossible to find through anything other than luck. This remains true even today. “American carriers are constantly surprised by small, diesel-electric submarines," in training exercises, Breemer says. "They're very quiet and hard to find."