One could consider the following 5 wars to be the most significant wars in history:
1. The Crusades (1095 – 1291)
2. The French Revolution (1789 - 1799)
3. The Mexican-American War (1846 - 1848)
4. World War I (1914 - 1918)
5. World War II (1939 - 1945)


Therefore, a good mnemonic might be:
1. 5 years before 11 (1100), lasted 4 years less than 200 (years)
2. 1 year shy of 1800, lasted a decade
3. 2 before 1850, lasted 2 years
4. 1914, +4
 5. 21 years later, +6

Please keep in mind that people remember things differently and the quality of a mnemonic is also dependent on your way of thinking.
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