Late 1850s - Mid-1860s: Denmark and Norway face unification issues, which eventually factor into the two wars in Northern Europe over the duchies of Schleswig and Holstein. At the same time, the United States faces its own unification issues, which erupt in a vicious civil war.

Today: The United States stands unified against a common enemy - terrorism - following vicious attacks by terrorist groups on the World Trade Center in New York and the Pentagon in Washington, D.C.

Late 1850s-Mid-1860s: Swedish pacifist Alfred Nobel performs a number of experiments on explosives. These experiments lead to the creation of nitroglycerin, which ultimately takes the portable form of dynamite. This powerful explosive is sometimes employed as a weapon.

Today: Terrorists make isolated attacks using bacteriological weapons like anthrax, and there is some concern that terrorists may resort to other, newer forms of warfare...