1800s: The 1842 Treaty of Nanking ends the Opium War between Great Britain and China and confirms the cession of Hong King to Great Britain.

Today: In 1997, Hong Kong is relinquished by the British and returned to Chinese rule.

1800s: Karl Marx and Friedrich Engels publish their Communist Manifesto in 1848; their work influences a number of world leaders who adopt several of its economic and social ideas.

Today: With the collapse of the Soviet Union, China remains the world's dominant communist power.

1800s: In 1844, George Williams organizes the Young Men's Christian Association (YMCA) of London, a service organization.

Today: The YMCA has expanded to include people of both sexes, all ages, and all religions as members. The World Alliance of YMCAs, headquartered in Geneva, Switzerland, has 30 million members in 110 countries.

1800s: In 1849, Elizabeth Blackwell becomes the first woman to earn a medical degree in the...