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Duration: 27 BC - 476 AD (Western Roman Empire); Greatest Achievements: Pax Romana, Roman Law, extensive road and aqueduct systems; Cause of Decline: Political corruption, economic troubles, military overspending, barbarian invasions

Duration: 16th century - 1997; Greatest Achievements: Industrial Revolution, establishment of parliamentary democracy, significant cultural and scientific contributions; Cause of Decline: Nationalist movements, costly wars, economic strain

Duration: 1206 - 1368; Greatest Achievements: Largest contiguous land empire, Pax Mongolica, Silk Road trade facilitation; Cause of Decline: Succession issues, division into khanates, military defeats

Duration: c. 1299 - 1922; Greatest Achievements: Architectural achievements like the Hagia Sophia, legal system, military dominance; Cause of Decline: Military defeats, loss of territories, economic stagnation

Duration: 550 - 330 BC; Greatest Achievements: Royal Road, Qanat irrigation systems, cultural diversity, and tolerance; Cause of Decline: The empire fell to Alexander the Great

Duration: 330 - 1453 AD; Greatest Achievements: Justinian Code, preservation of classical knowledge, Hagia Sophia; Cause of Decline: Constant wars, Fourth Crusade, fall to the Ottoman Turks

Duration: 1492 - 1976; Greatest Achievements: Global empire, spread of Christianity, vast colonial wealth; Cause of Decline: Independence movements, costly wars, economic troubles

Duration: 1428 - 1521; Greatest Achievements: Complex calendars, Nahuatl writing system, chinampa agriculture; Cause of Decline: Spanish conquest led by Hernán Cortés

Duration: 661 - 750 AD; Greatest Achievements: Largest Islamic empire, Arabic became lingua franca, architectural achievements like the Great Mosque of Damascus; Cause of Decline: Abbasid Revolt and internal family disputes

Duration: 1368 - 1644; Greatest Achievements: Construction of the Forbidden City, Zheng He's voyages, restoration of the Great Wall of China; Cause of Decline: Peasant revolts, Manchu invasion, fiscal problems

Duration: 1644 - 1912; Greatest Achievements: Expansion to greatest size, prosperity during High Qing era, strong imperial bureaucracy; Cause of Decline: Opium Wars, internal rebellions, external pressures leading to the Xinhai Revolution

Duration: 322 - 185 BC; Greatest Achievements: Unification of the Indian subcontinent, Ashoka's promotion of Buddhism, political and military administration; Cause of Decline: Succession struggles and regional powers rise after Ashoka's death

Duration: c. 320 - 550 AD; Greatest Achievements: Golden Age of India, advancements in science, math, astronomy, religious tolerance; Cause of Decline: Huna invasions, weak rulers, provincial powers

Duration: 962 - 1806; Greatest Achievements: Influential in European politics, religious affairs, diverse cultural developments; Cause of Decline: Power struggles, Napoleonic Wars, dissolution by Francis II

Duration: 800 - 888 AD; Greatest Achievements: Carolingian Renaissance, standardized Christian worship, military campaigns; Cause of Decline: Treaty of Verdun, internal divisions, Viking invasions

Duration: 1721 - 1917; Greatest Achievements: Expansion to become one of the largest empires by land area, cultural expansion, industrialization; Cause of Decline: Military defeats in WWI, economic hardship, Russian Revolution

Duration: c. 1438 - 1533; Greatest Achievements: Large road system, precise stone architecture, agricultural advancements; Cause of Decline: Spanish conquest led by Francisco Pizarro, internal conflict

Duration: 16th century - 1980; Greatest Achievements: Second largest colonial empire at its peak, spread of French culture and language, significant architectural and scientific contributions; Cause of Decline: Decolonization after WWII, nationalist movements, wars of independence

Duration: 1868 - 1945; Greatest Achievements: Meiji Restoration, rapid industrialization, military expansion in Asia; Cause of Decline: Defeat in WWII, atomic bombings, unconditional surrender

Duration: 750 - 1258; Greatest Achievements: Islamic Golden Age, advancements in knowledge and culture, centralized administration; Cause of Decline: Internal fragmentation, Mongol invasion and sacking of Baghdad

Duration: 16th century - 1975; Greatest Achievements: Maritime dominance in the 17th century, economic power, significant artistic contributions; Cause of Decline: Rise of British and French power, nationalist movements, costly wars

Duration: 1867 - 1918; Greatest Achievements: Political power in Europe, cultural diversity, influence on classical music; Cause of Decline: Nationalist tensions, defeat in WWI, dissolution after the Treaty of Saint-Germain

Duration: c. 1230 - 1600; Greatest Achievements: Wealth from gold and salt trade, Timbuktu as center of Islamic scholarship, Mansa Musa's pilgrimage; Cause of Decline: Internal strife, lack of strong leadership, invasion by Moroccans

Duration: 1415 - 1999; Greatest Achievements: Global trade network, Age of Discovery, cultural exchanges via colonization; Cause of Decline: Overextension, military overcommitment, competition from other empires