In the 19th century, the major European powers had gone to great lengths to maintain a balance of power throughout Europe, resulting by 1900 in a complex network of political and military alliances throughout the continent. Beginning in 1815, the Holy Alliance between Prussia, Russia, and Austria formalized what had been a strictly familial support network. The House of Habsburg, the House of Hohenzollern, and the House of Romanov had long intermarried,...