World War I and World War II were two of the most significant global conflicts in modern history, each with its own series of episodes and consequences.

**World War I:**
1. **Assassination of Archduke Franz Ferdinand:** The assassination of Archduke Franz Ferdinand of Austria-Hungary in 1914 triggered a chain of events leading to the outbreak of war.
2. **Trench Warfare:** Much of the war on the Western Front was characterized by brutal trench warfare, with soldiers enduring horrific conditions.
3. **War of Attrition:** Both sides engaged in a war of attrition, resulting in millions of casualties.
4. **Treaty of Versailles:** The war ended with the Treaty of Versailles in 1919, imposing heavy reparations and territorial losses on Germany, setting the stage for future tensions.

**World War II:**
1. **Rise of Fascism:** The rise of fascist regimes in Germany, Italy, and Japan contributed to the outbreak of war.
2. **Invasion of Poland:** Hitler's invasion of Poland in 1939 led to the formal start of World War II.
3. **Blitzkrieg Tactics:** Germany's rapid conquests in Europe, using blitzkrieg tactics, led to the fall of multiple countries.
4. **Holocaust:** The systematic genocide of six million Jews by the Nazis, known as the Holocaust, was one of the darkest episodes of human history.
5. **Atomic Bombings:** The United States dropped atomic bombs on the cities of Hiroshima and Nagasaki in 1945, leading to Japan's surrender and the end of the war.

In terms of accountability, after World War II, the Allies held the Nuremberg and Tokyo trials to prosecute war criminals, including leaders and military officials responsible for atrocities. These trials established important precedents for international law and accountability for war crimes, crimes against humanity, and genocide. However, achieving accountability for the complex web of actors involved in wartime atrocities remains a challenging and ongoing endeavor.