1. WWI
2. WWII

The Allies were terrified that Hitler would develop an atomic bomb. So they started the Manhattan Project as part of a race to create the world's first nuclear weapons.

1. WWI
2. WWII

In WWI, Japan joined the fight on the side of Western Allies. A generation later, of course, Japan would vow to kill Allied soldiers wherever they met in battle.

1. WWI
2. WWII

In 1914, a gunman murdered Archduke Francis Ferdinand of Austria. The assassination set into motion an avalanche of events that resulted in The Great War.

1. WWI
2. WWII

After WWII, the Soviet Union and United States (and some of its allies) settled into the Cold War. It was a geopolitical stalemate of sorts, in which both sides maneuvered for the best political and economic advantages.

1. WWI
2. WWII

In WWII, the Nazis used "blitzkrieg" or lightning war, which consisted of fast-moving assaults meant to overwhelm enemy lines.

1. WWI
2. WWII

Mussolini was a fascist leader in Italy during WWII. He buddied up to Hitler and hoped to become a political powerhouse once the Third Reich had taken over Europe.

1. WWI
2. WWII

Just before the end of WWII, Franklin Roosevelt's health finally failed and he passed away April 12, 1945. That left Harry Truman as the man in charge when Japan surrendered on September 2, 1945.

1. WWI
2. WWII

David Lloyd George served as Britain's Prime Minister from 1916 to 1922. During WWII, Winston Churchill became Prime Minister and emerged as a political legend for the ages.

1. WWI
2. WWII

WWI, The Great War, "The War to End All Wars" was a devastating conflict that altered the history of humankind. And yet, 20 years later, we did it all over again.

1. World War I
2. World War II

WWI was also called The Great War. It was supposed to be the last big war fought by humankind but that concept clearly didn't pan out.

1. WWI
2. WWII

The Allies were victorious in WWI and forced Germany to sign the Versailles Treaty, which came loaded with all sorts of humiliating conditions -- the conditions that later contributed to the start of WWII.

1. WWI
2. WWII

WWI churned the world for four years. WWII swept the globe for six years, from 1939 to 1945.

1. WWI
2. WWII

In WWII, the Nazis struck out for Moscow and stopped at Leningrad along the way. The Soviets refused to relent, resulting in the Siege of Leningrad, which lasted 900 days and resulted in scenes of cannibalism as city residents starved.

1. WWI
2. WWII

In WWI, Turkey was one of the Central Powers, along with Bulgaria, Austria-Hungary, and of course, Germany.

1. WWI
2. WWII

In late 1944, the Germans launched one last major offensive on the Western Front, resulting in the bloody Battle of the Bulge which spanned from December 16, 1944 to January 25, 1945. American forces suffered mightily in an effort to drive back the Germans with 89,000 casualties.

1. WWI
2. WWII

There's no real comparison here. WWII killed around 60 million soldiers and civilians worldwide -- some historians think the number may be as high as 85 million. In comparison, around 17 million people died during WWI.

1. WWI
2. WWII

WWII was a battle of competing ideologies. Democracy, fascism, and communism all faced off in an epic (and bloody) showdown.

1. WWI
2. WWII

The Marshall Plan pumped billions of dollars into the area in an effort to rebuild cities and restore lives of millions of war-scarred civilians.

1. WWI
2. WWII

During WWI, armies had newfangled and high-powered weapons matched versus outdated battlefield tactics resulting in bloody slaughters. So generals resorted to stagnant trench warfare to cut down on casualties.

1. WWI
2. WWII

Operation Barbarossa was the German invasion of the Soviet Union in WWII. Hitler's decision to launch a war against the Soviets was a key factor in the downfall of the Third Reich.

1. WWI
2. WWII

In WWI, there were about 7 million civilian deaths compared to 10 million military deaths. During WWII, about 60 million people died, and the majority of them were civilians.

1. WWI
2. WWII

The Soviets joined Germany in the pillaging of Poland at the start of WWII. Later, the Soviets and Germans would become enemies.

1. WWI
2. WWII

In 1935, Europe was struggling to maintain peace. Then Italy decided to invade Ethiopia, and suddenly, military aggression didn't seem like such a bad option.

1. WWI
2. WWII

In early 1941, the United States enacted the Lend-Lease Act, which authorized material support for the Allies. But the U.S. didn't send troops into combat until after the Japanese attacked Pearl Harbor in December of the same year.

1. WWI
2. WWII

After WWI, world leaders created the League of Nations, a political body intended to prevent future large-scale armed conflicts.

1. WWI
2. WWII

Flamethrowers were used occasionally in WWI and they became much more common (to the chagrin of soldiers everywhere) during WWII.

1. WWI
2. WWII

Both sides used a lot of aircraft during WWI, primarily for surveillance purposes. In WWII, however, aircraft became potent offensive weapons.

1. WWI
2. WWII

In WWI, Russia opted to attack Germany, an event that helped the war spiral out of control. During the Second World War, of course, Germany flipped the script and attacked Russia, much to Josef Stalin's chagrin.

1. WWI
2. WWII

The Eastern Front was home to some of the bloodiest battles of WWII. There, the Soviets clung to their homeland, finally beating back the Nazis and winning The Great Patriotic War.

1. WWI
2. WWII

In part to counter the horrors of trench warfare, tanks became more common during WWI. At first, they were slow and unwieldy, but by the end of the war, some of these machines were quite capable and deadly.