The most influential book on war was unfinished, and his wife published it
Carl von Clausewitz joined the Prussian army at twelve, was captured by Napoleon's forces at 26, and switched to the Russian side rather than serve alongside France. His great treatise On War lay incomplete at his death in 1831. Marie von Clausewitz edited and published it.
Born in 1780 at Burg bei Magdeburg, the youngest of four sons, he grew up in a family claiming noble descent that scholars doubt. His father had been a lieutenant under Frederick the Great before taking a minor tax post. Carl enlisted as a lance corporal at twelve, fought in the Rhine campaigns of 1793 and 1794, and in 1801 entered the war academy in Berlin, where he probably studied Kant and Fichte and won the backing of Gerhard von Scharnhorst. With allies such as Hermann von Boyen he helped Scharnhorst reform the Prussian army after 1807.
At Jena-Auerstedt in October 1806 he was among roughly 25,000 prisoners taken as Prussia's army collapsed, and spent 1807 to 1808 held in France with his prince. In 1810 he married Countess Marie von Brühl, a well-connected, highly educated woman who shaped his career and thinking. When Prussia was forced into alliance with Napoleon, he left for Russian service, fought at Borodino in 1812, and helped negotiate the Convention of Tauroggen, which opened the way to the coalition that eventually beat Napoleon. Back in Prussian uniform in 1815, his corps fought heavily outnumbered at Wavre, keeping reinforcements from reaching Napoleon at Waterloo.
His ideas centre on friction and uncertainty. Under the fog of war, commanders act on incomplete, doubtful and often wrong information amid fear and excitement, so they must decide fast. Unlike the early Antoine-Henri Jomini, he denied that war could be reduced to geometry, maps and graphs, and he trusted history as a check on neat abstractions. He stressed psychological, or in his terms moral, forces and the political purpose behind fighting, summed up in his line that war continues policy by other means.
Even his name is disputed. He spelled Carl with a C to align with classical tradition, and sources give his middle names variously; modern scholars trust his tombstone, which reads Carl Philipp Gottfried. After the wars he ran the war academy until 1830, then returned to active duty and died the following year.
Source: Carl von Clausewitz
