Modern conventional weapons, deadly as they are, have no monopoly on terrorizing soldiers and civilians. Many military innovations of the past scared the enemy senseless — especially when only one side got to use them.

"When you defeat someone psychologically, that's really how you win battles," says Pentagon spokesman and artillery officer Lt. Col. Mark Wright. "If [enemy forces] think they've been beaten, they're going to turn and run."

Here, then, are five of the most terrifying conventional weapons of all time. Each was effective because it was a surprise introduction to a conflict, permitted essentially no defense, and only one side got to use it — though, as we'll see, that didn't always guarantee victory.

• Click here for more photos.

• Click here for's Patents and Innovation Center.

Greek fire: Perhaps the single most effective secret weapon ever, this highly flammable liquid won countless sea battles for the Byzantine Empire over eight centuries.

Some historians argue it was crucial to letting the continually dwindling empire — by the end, just the city of Constantinople — survive as long as it did.

The composition of Greek fire was a highly guarded state secret, one that was lost forever when the Ottoman Turks finally captured Constantinople in 1453. Modern historians speculate it contained some sort of petroleum, or possibly phosphorus.

It burned on water, could be shot in any direction and was packaged into grenades and cannonballs, helping the Byzantines fend off Arabs, Vikings, Crusaders, various Turks and all manner of Italians.

It could also be hurled by catapults over city walls or dumped on attackers trying to climb them.

But best of all were the specialized bowsprits built into Byzantine naval vessels, which made it appear as if the deadly flames were shooting out of the mouths of terrifying metal dragons, lions or other fierce animals.

It wasn't "so much the lethality as the fear of the flames," explains Wright. "It was very effective at sea — you could throw it on the enemy's ships and burn them down."

• Click here for a brief video clip about Greek fire.

Napalm: This mixture of gasoline and a thickening agent is similar to Greek fire and is best known for use against Viet Cong supply lines by American forces during the Vietnam War. It was actually first used toward the end of World War II against German and Japanese positions.

American pilots noticed its effectiveness in terrifying the enemy during the Korean War, remarking that North Korean troops would surrender to aircraft following napalm attacks. The French used it against the Viet Minh, the Viet Cong's predecessors, later in the 1950s.

Napalm is so effective because it sticks to anything it hits and burns at about 1,800 degrees Fahrenheit for up to 10 minutes. There's little soldiers or civilians hit by it can do except hope to survive.

• Click here for film footage of a famous mistaken South Vietnamese napalm attack against civilians in 1972.

The Gatling gun: Briefly used by Union forces during the American Civil War, the first mass-production machine gun came into wide use in the 1870s and 1880s by European armies spreading their power across Africa and Asia in the race for empire.

The British used it to mow down Zulu tribesmen; the Russians did the same to nomadic horsemen on the Central Asian steppe.

It was less effective against modern armies. Much too heavy to be carried by hand, the Gatling gun was mounted on wheels like a cannon and cranked by the operator to turn the six firing barrels around a central axis as a second man stood to hand-feed the bullets.

This exposed its crew to rifle fire, and the huge amount of gunsmoke it gave off meant the crew often couldn't see what it was shooting at.

By the turn of the 20th century, the Gatling gun was being replaced by the more efficient, fully automatic Maxim gun and its derivatives, all of which saw much use in World War I.

But the Gatling principle came back after World War II, when American weapons designers mounted rapid-fire revolving-barrel guns on aircraft; the AC-130 Spectre and Spooky gunships and A-10 "Warthog" ground-support aircraft both use modern Gatling guns.

• Click here to see an 1874 model Gatling gun in action.

• Click here to see the firing of a modern Gatling gun mounted in the nose of an A-10.

The Paris gun: This long-barreled artillery piece deployed by the Germans during World War I killed relatively few people, but because its range was so long — 75 to 80 miles — it told the citizens of Paris, far from the front lines, that the war could get to them too.

By the standards of the time, the Paris gun was truly a superweapon. Its barrel was over 100 feet long, it was mounted on a railway carriage manned by naval gunners and its shells tore away so much of the rifling during firing that sequentially wider shells had to be used until the barrel was completely torn up and had to be replaced.

Once fired, each shell reached an altitude of 25 miles, the edge of the stratosphere, where frictional forces were minimal, enabling them to travel further than any previous artillery weapon — so far that German gunners had to adjust their aim to compensate for the rotation of the Earth.

The first shells began landing in Paris in mid-March 1918, and locals, having heard neither aircraft nor guns, initially assumed they'd been bombed by a high-altitude blimp. The biggest loss of life came on March 29 when a shell hit a church during Good Friday services; 88 were killed.

"For a city the size of Paris, it was just a terror weapon," says Wright. "It didn't paralyze the city and didn't shut anything down, but it was a terrifying weapon to be able to reach out that far."

Fortunately for the French, the Paris gun could fire only about 20 shells per day and constantly needed to be overhauled. By the time it had to be moved out of range in the face of advancing Allied forces in August 1918, it had fired about 350 shells in total, killing 250 people over five months.

Although the Paris gun was dismantled and destroyed by the Germans to prevent its capture by the Allies, enough was known about it so that it influenced later designs. One joint U.S.-Canadian descendent project during the 1960s was used for low-cost ballistics tests.

That project's main designer, Gerald Bull, later became a renegade weapons peddler, selling long-range howitzers to South Africa and Iraq among others.

He was assassinated in Brussels in 1990, possibly by Israel's Mossad intelligence agency or by its Iranian counterpart, as he was trying to develop a 500-foot-long gun for Saddam Hussein.

The V-2 rocket: Germany's famed "revenge weapon" was the first ballistic missile to be used in wartime; its predecessor, the V-1 or "buzz bomb," was the first effective cruise missile. The entire early U.S. space program, and most of the Soviet one as well, was based on the V-2.

Early research on what later became the V-2 began long before the war. The first successful launch was in late 1942, but that was a fluke because almost all the other launches blew up in mid-air.

Hitler was rightly less than impressed with early tests, dismissing the V-2 as too expensive and inaccurate, but the airburst problem was eventually solved and mass production began in early 1944 at a slave-labor factory deep in central Germany.

Nearly 2,000 had been built before the first V-2 was launched into Paris on Sept. 8, 1944, causing little damage. Later that same day, two landed on London, killing three people, and for the next few months, the rocket war was on.

The V-2 was technically astonishing. Its liquid-fuel engines cut off about a minute after launch, their timing precisely controlled by an analog computer.

From then on it was mostly physics as the 50-foot, 27,000-pound rocket soared to the edge of space 55 miles up, then came down so fast it broke the sound barrier.

"The old V-1 [missiles] could be shot down," explains Wright. "The V-2 traveled at supersonic speeds and couldn't be tracked, couldn't be shot down."

Some V-2 launch teams were even equipped with long-range radio transmitters that pointed straight at the target, allowing the missile's receivers to home in by adjusting its four tailfins.

Its range was about 200 miles, and specially designed trailers meant it could be launched by mobile teams from anywhere.

Popular culture has fixed the notion of the V-2 as a weapon directed mainly against London, but it was the strategically key Belgian port of Antwerp that took even more hits, especially after the Germans failed to retake the city during the Battle of the Bulge at the end of 1944.

Once a V-2 was launched, there was little that could be done in defense. Early attempts to bombard them with anti-aircraft fire as they came down proved unsuccessful; there is one account of an American bomber downing one it happened to catch upon launch.

Countermeasures focused on destroying the launch facilities, but many were operated by highly mobile teams hiding in forests. (Submarine launches, planned for use against American coastal cities, passed tests but never came to fruition.)

Allied ground advances ended the V-2 launches in March 1945, when the rockets were either captured or moved out of range.

More than 3,000 V-2s had been launched against Western targets, and more than 7,000 people had died as a result.

But the ratio of deaths to losses made the V-2 a failure in terms of costs and benefits, especially since it did nothing to turn the tide of the war.

Far less lucky than the residents of London and Antwerp were the estimated 20,000 Russian, Polish and French slave laborers who were starved, hanged or simply worked to death as they built V-1 and V-2 missiles at the Mittelbau-Dora slave-labor camp in the central German mountains.

The real beneficiary of the V-2 program was the U.S. military, which spirited dozens of its scientists and engineers , including project leader Wernher von Braun, across the Atlantic, where they became the foundation of the U.S. ballistic-missile program and then later of NASA.

• Click here for German footage of V-2 test launches.

• Click here for U.S. Army footage of the liberation of Mittelbau-Dora.