Ahead of the official celebration, volunteer actors put on British military uniforms and took a ride around the ceremony in restored vintage military vehicles. On D-Day, American, British and Canadian forces combined landed around 156,000 troops on the beaches of Normandy by sea and by air.

British jets flew over the ceremony at Portsmouth on Monday. More than 4,000 aircraft were engaged in the invasion of Normandy on between June 5-6.

A 94-year-old British veteran waves during a ceremony held in Caen, a small city in Normandy. He was part of a group of veterans who retraced the journey across the English Channel to commemorate the D-Day anniversary. One former American paratrooper, 97-year-old Tom Rice, even landed again by parachute — although this time with a tandem partner.

US President Donald Trump, German Chancellor Angela Merkel, Dutch Prime Minister Mark Rutte and Canadian Prime Minister Justin Trudeau, were among the handful of world leaders who gathered in Portsmouth. Trump read from a prayer given by President Franklin D. Roosevelt on June 6, 1944. "A struggle to preserve our republic, our religion and our civilization and to set free a suffering humanity."

Queen Elizabeth was 19 years old on June 6, 1944. During a short speech on Monday, the queen said: "that generation — my generation — is resilient." She also quoted from a broadcast given by her father, King George VI, after the invasion, calling for "courage and endurance ... a revival of spirit, a new unconquerable resolve."

A group of D-Day veterans, many over 90 years old, appeared on stage during the ceremony in Portsmouth to remember the soldiers who did not survive. More than 4,400 Allied servicemen were killed on D-Day alone.

It was announced that the night before Monday's ceremony in Portsmouth that the leaders of all 16 countries involved in World War II had signed an symbolic peace proclamation to ensure that the horrors of the war are never repeated.

The Normandy invasion is known historically as D-Day, but it remains unclear what exactly the "D" means. Whether it's simply a signifier for "Day," or it means "Decision," one thing is for certain. It was the beginning of one of the most significant battles in military history.

The five sectors of the 80-kilometer (50-mile) stretch of Normandy coast where the landings happened were all given different names: Utah, Omaha, Gold, Sword, and Juno. Apart from the US, 13 other nations were involved in the invasion. Troop contingents were provided by the Americans, British, Polish, Canadians, French, Greeks, Czechs and Australians.

The commander of the Allied forces in northern Europe was General Dwight Eisenhower, who would go on to become the 34th president of the United States. A few months prior to D-Day, Eisenhower had successfully led landings in Sicily and mainland Italy.

Just before the planned start of Operation Overlord on June 5, storms broke over Normandy, forcing Allied commanders to push the invasion back a day. The largest amphibious landings in military history got underway on June 6. The weather was not ideal, but allies feared Germans would learn of their plans if the delay continued.

Around 156,000 soldiers reached land on D-Day. On five different locations on the beach, they stormed the "Atlantic Wall," where German Wehrmacht soldiers were perched in fortifications that had been built in anticipation of an assault. The allied troops were forced to run unprotected, first through water and then onto the beach, all the while under German fire.

Before the D-Day Invasion began, paratroopers had touched ground early on June 6 in a bid to secure key positions behind the Atlantic Wall. The troopers were camouflaged (shown here); they also used war paint and Mohawks to frighten the enemy.

At first, the beaches of Normandy were bombed from the air by the Allies. After the beach had been secured, over 1,000 warships and some 4,200 landing crafts converged on the coast. Further reinforcement was provided by thousands of planes and tanks.

One of the reasons why "Operation Overlord" was so successful was that the German military command simply wasn't expecting it - at least not in Normandy. The Allies duped the Nazis into thinking they would land at Calais, near the Belgian border, and at a date later than June 6.

Many leading NS officers had absolutely no idea D-Day was going to happen when it did. Field Marshall Erwin Rommel, for instance, was celebrating his wife's 50th birthday in the south of Germany when the invasion started. (Pictured above: Wehrmacht divisions at Normandy in 1940)

When the invasion got underway, Adolf Hitler was sleeping in Obersalzberg. His officers didn't dare wake him, and when they did (at 10 in the morning), the Nazi leader was in surprisingly good spirits. After being briefed, he is said to have exclaimed: "The news couldn't be better!" Hitler was apparently pleased that the Allies were "finally in a place where we can defeat them."

Even if the Normandy landings were perhaps the decisive battle for the Allies, the final victory came at heavy price. It still took 11 months for peace to be declared in Europe, and many of the soldiers who took part in the invasion were shipped straight to the Asia Pacific shortly thereafter. The war lasted until September 2, 1945, when Japan capitulated.

Around 57,000 Allied soldiers lost their lives in Operation Overlord, which started on D-Day and continued until the end of August. Another 155,000 were injured and 18,000 reported missing. German losses rounded out at about 200,000. Each year on June 6, there is a ceremony of remembrance at Normandy. Heads of state and government and many veterans often make the trip to the French coast.

In 2004, German Chancellor Gerhard Schröder became the first German leader to take part in the ceremonies at Normandy. Chancellor Gerhard Schröder, shown embracing France's Jacques Chirac, chose his words carefully for the event: "We will never forget the victims." Schröder added: "It's not the old Germany of those dark years that I am representing today."