World War I, also known in its time as the "Great War" or the "War to End all Wars", was an unprecedented holocaust in terms of its sheer scale. Fought by men who hailed from all corners of the globe, it saw millions of soldiers do battle in brutal assaults of attrition which dragged on for months with little to no respite. Tens of millions of artillery shells and untold hundreds of millions of rifle and machine gun bullets were fired in a conflict that demonstrated man's capacity to kill each other on a heretofore unprecedented scale, and as always, such a war brought about technological innovation at a rate that made the boom of the Industrial Revolution seem stagnant. The enduring image of World War I is of men stuck in muddy trenches, and of vast armies deadlocked in a fight neither could win. It was a war of barbed wire, poison gas, and horrific losses as officers led their troops on mass charges across No Man's Land and into a hail of bullets.

If trench warfare was an inevitability during the war, it is only because the events leading up to the First Battle of the Marne were quite different. The armies at the beginning of the war moved quickly through the land, but the First Battle of the Marne devolved into a bloody pitched battle that led to the construction of trenches after the Germans retreated, blocked in their pursuit of Paris. When the aftermath disintegrated into a war between trenches, some Germans thought they had the upper hand since they were occupying French territory, but with fewer soldiers than the combined Allied nations and fewer resources and supplies, it was possibly only a matter of time before they were ultimately defeated. The commander of the German armies, General Helmuth von Moltke, allegedly said to Kaiser Wilhelm II immediately after the First Battle of the Marne, "Your Majesty, we have lost the war." Winston Churchill himself would later reference that anecdote, writing, "Whether General von Moltke actually said to the Emperor, 'Majesty, we have lost the war, ' we do not know. We know anyhow that with a prescience greater in political than in military affairs, he wrote to his wife on the night of the 9th, 'Things have not gone well. The fighting east of Paris has not gone in our favour, and we shall have to pay for the damage we have done.'"

One of the most famous people in the world came to tour the city of Paris for the first time on June 28, 1940. Over the next three hours, he rode through the city's streets, stopping to tour L'Opéra Paris. He rode down the Champs-Élysées toward the Trocadero and the Eiffel Tower, where he had his picture taken. After passing through the Arc de Triomphe, he toured the Pantheon and old medieval churches, though he did not manage to see the Louvre or the Palace of Justice. Heading back to the airport, he told his staff, "It was the dream of my life to be permitted to see Paris. I cannot say how happy I am to have that dream fulfilled today." Four years after his tour, Adolf Hitler would order the city's garrison commander, General Dietrich von Choltitz, to destroy Paris, warning his subordinate that the city "must not fall into the enemy's hand except lying in complete debris."

Of course, Paris was not destroyed before the Allies liberated it, but it would take more than four years for them to wrest control of France from Nazi Germany after they took the country by storm in about a month in 1940. That said, it's widely overlooked today given how history played out that as the power of Nazi Germany grew alarmingly during the 1930s, the French sought means to defend their territory against the rising menace of the Thousand-Year Reich. As architects of the most punitive measures in the Treaty of Versailles following World War I, France was a natural target for Teutonic retribution, so the Maginot Line, a series of interconnected strongpoints and fortifications running along much of France's eastern border.