When German Panzers rolled into the French coastal region around Calais in late May 1940, their crews could stare across the English Channel at the White Cliffs of Dover, just 20 miles away.

The United Kingdom had not faced a hostile enemy across the Channel since the Napoleonic Wars. In that interval, the maximum range of heavy artillery had increased dramatically. Hitler was alert to the opportunity this afforded him for his planned invasion of Britain, dubbed Operation Sea Lion.

“Strong forces of coastal artillery must command and protect the forward coastal area,” Hitler wrote in a July 16 invasion plan. He wanted the batteries not only to protect his invasion force from the Royal Navy, but to bombard the British defenders on the opposite shore.

The big guns began to arrive a week later, accompanied by work crews to build giant concrete casements to protect them from counter-bombardment. The best were turreted naval guns originally designed for use on battleships that could track and fire rapidly to hit moving ships.

At Cape Gris Nez, the Germans mounted four intimidating 380-millimeter SK34 naval guns of Battery Todt in enormous concrete casemates. Nearby were the four 280-millimeter guns of the Grosser Kurfurst battery.

On Cape Blanc Nez, the beach immediately west of Calais, three 406-millimeter “Adolf Cannons” were installed in casemates shielded by 13 feet of concrete. These could lob one-ton shells up to a distance of 34 miles.

Four more turreted coastal guns were installed around Calais, and three 305-millimeter naval guns with a 32 mile range were deployed near the city of Boulogne to the south.

The Wehrmacht also brought eight railway guns and 40 army siege guns into the Calais region. These ranged from 21 to 28 centimeters in caliber. However, they lacked the ability to rapidly adjust fire to strike moving maritime targets.

At 11 a.m. on Aug. 12, 1940, a shell exploded in Dover, damaging four houses. It was the first of thousands of enormous siege shells that would land in the coastal town over the next four years.

After giving his “We shall fight on the beaches” speech in June, Churchill had to reckon with the fact the British military initially had no heavy coastal guns defending the beach at Dover.

On July 10, construction began on a concrete emplacement behind the village of Saint Margaret’s-at-Cliffe, northeast of Dover. A 60-ton armored battleship mount arrived six days later, as well as a spare Mark VII 14-inch gun from the stock reserved for King George V-class battleships.

As construction proceeded, Spitfires and Hurricanes of the Royal Air Force fought daily battles overhead against swarms of Luftwaffe bombers in the Battle of Britain. In mid-August, German Ju-87 Stukas dropped 30 bombs on the construction site, but failed to inflict much damage. Flak from 40-millimeter Bofors cannons and Lewis machine guns shot down two in return.

A new Royal Marine Siege Regiment manned the emplacement, dubbed Winnie after the British prime minister. Winnie was connected by a railway tunnel to a subterranean ammunition depot, and had a separate plotting room and its own medical facilities.

The emplacement was swathed in camouflage netting, and two additional dummy batteries with inferior camouflage existed to draw German fire. Its 14-inch gun could propel a 1,590-pound shell up to 27 miles when using extra charge.

Winnie fired the first shell to cross from England to France on Aug. 22, 1940. Targeting one of the German gun batteries, it caused minor damage and wounded a corporal.

The far more numerous German guns soon responded with a withering barrage. According to German records, Winnie fired 25 shells in September, with little effect besides wounding a French farmer. Four shells fired in October caused a Luftwaffe mechanic to lose an arm.

Over time, whenever one side’s guns fired at passing ships, the other would retaliate. The area around Dover and the nearby town of Folkestone became known as Hellfire Corner — with the civilian inhabitants of Dover the principal victims.

The German guns often deliberately targeted civilian areas in the town to discourage British counter-battery fire. Dover’s population fell to half the pre-war level. The bombardments inspired even greater dread then air attacks because incoming shells could not be heard until after they had struck their targets.

Over the course of four years, German shelling killed 216 civilians and damaged more than 10,000 homes in Dover.

In February 1941, Winnie was joined by her less reliable-sister gun Pooh, situated slightly to the east of St. Margaret. The guns became popular with visiting dignitaries and wartime propaganda reels. However, they were so slow to fire that they could only hit immobile targets.

Lacking any form of radar targeting, the crews relied on fighter planes to spot the impact of the shells and correct their aim. The heavy charges necessary to shoot at long range also wore through the barrels rapidly, degrading range and accuracy, and requiring frequent changes for repairs.

A report in 1943 observed, “As you can see, there are no real grounds for retaining this Regiment under present conditions other than the Prime Minister’s personal affection for these pieces.”

Larger and more effective guns were coming, however, including Clem and Jane, the first named after politician — and later prime minister — Clement Atlee, and the second after a racy ingénue in a Daily Mirror comic strip. These larger, turreted 15-inch guns of the Wanstone Battery were on a reverse slope just inland of the White Cliffs of Dover, and could maintain a higher rate of fire to hit German ships.

Minefields and supplies of small arms were also deployed around the gun in event of an invasion.

Three World War I-era 13.5-inch railway guns named Piece Maker, Scene Shifter and Gladiator also contributed their firepower, popping out of the Guston railway tunnel near Martin Mill station to unleash their shots then ducking back inside to avoid retaliation. Counter-battery fire was a real threat, as shell splinters mortally wounded a crew member on Piece Maker.

A fourth railway gun, Boche Buster, mounted a massive 18-inch gun. Deployed in case of a German invasion, it lacked the range to reach France and thus never fired a shot in anger.

The most effective British coastal guns, however, were four Mark IX 9.2-inch guns deployed to the South Foreland battery which became active in July 1941. These 11-meter long pieces, which relied more on camouflage than concrete for defense, had a shorter maximum range of 21 miles, but benefited from newly installed K-Band coastal defense radars capable of tracking and targeting ships.

Smaller six-inch anti-shipping batteries and eight-inch dual-purpose guns were also installed at Fan Bay in the Port of Dover, and at Lydden-Spout and Hougham in the direction of Folkstone.

The prize target of any coastal gun is an enemy capital ship. The German guns in Calais never had a decent shot at one — but the Dover guns received their one and only chance during the infamous Channel Dash on Feb. 12, 1942.

The Kriegsmarine in World War II could not shift its surface warships between the Mediterranean and the North Sea without passing either through the straits of Dover or taking the long way around England. Both routes exposed its surface ships to detection and overwhelming attack.

Fearing a British invasion of Norway, however, Hitler decided to rush the battleships Scharnhorst and Gneisenau and the heavy cruiser Prinz Eugen at maximum speed northward through the Channel.

British radar detected the German warships at noon, but poor weather obscured them from view. Only the radar-guided South Foreland battery had any means of targeting the warships. The 9.2-inch guns unleashed 33 shells in six minutes of rapid fire, attempting to use the radar returns of their shells splashing in the water to correct their aim.

The guns in Calais responded with counter-battery fire even as the German capital ships slipped out of range. Afterward, the South Foreland battery estimated it had made four hits — but in reality, the shots had literally missed by a mile. The German capital ships proceeded to blast their way through a hasty air and sea attack that went miserably for British forces.

But hitting any type of ship was rare for the guns on both sides. This was not for lack of trying — the Calais guns regularly sniped at British coastal convoys, and Dover guns fruitlessly attempted to swat German motor torpedo boats. However, the great range and the fact that the boats were moving targets caused the vast majority of shells to miss.

The Fan Bay Battery’s moment of glory came on August 1942 when its six-inch Mark VII guns sank an R-Boat — a 134-foot German minesweeper. Heavier British guns managed to sink two small transports in 1943, and two larger vessels and a torpedo boat in 1944, totaling 17,000 tons.

The German guns didn’t claim their first victim until June 6, 1944, D-Day, striking the Lend-Lease Liberty ship SS Sambut loaded with tanks, ammunition and trucks. The onboard vehicles, preloaded with gasoline and gelignite, caught fire, forcing the crew and passengers to abandon ship.