On 14 October 1806, in two battles fought the same day at Jena and Auerstedt, Napoleon broke the army that Frederick the Great had left to Prussia. Two weeks later, near Prenzlau, the French captured Prince August, a nephew of Frederick, together with his adjutant, the twenty-six-year-old Carl von Clausewitz. He spent the next months as a captive in France, and the rest of his life working out why such a thing could happen. His answer is the idea he is remembered for: war is not a craft with rules of its own but the continuation of politics by other means, so the purpose a state pursues decides how it should fight, how hard and for how long.
He had grown up inside that army. The son of an excise collector in Burg, from a family of pastors, he joined an infantry regiment in 1792 at the age of twelve and was commissioned the next summer at the siege of Mainz, where Prussia was fighting revolutionary France. In 1801 he entered the war school in Berlin. Its director, Gerhard von Scharnhorst, became his friend and, in the words of the German national biography, the teacher who set the direction of his thinking. After the defeat, Scharnhorst led the rebuilding of the army, and Clausewitz worked under him in the war ministry from 1809. In December 1810 he married Countess Marie von Brühl.
The reformers wanted Prussia to fight Napoleon again, but in February 1812 the king agreed instead to send 20,000 men to march with Napoleon against Russia. Clausewitz wrote a memorandum, the Political Declaration, arguing that a people owes nothing higher than the defence of its dignity and freedom. Then he resigned and joined the Russian army. The German national biography calls it the most consequential decision of his life, taken alone, on his own conscience. That September he was at Borodino, the great battle before Moscow, on the staff of General Uvarov's cavalry. At the end of the year he was among the officers who talked Ludwig Yorck, commander of the Prussian corps marching with Napoleon's army, into the Convention of Tauroggen. Signed without the king's consent, it took the corps out of the war and began Prussia's turn against France. The king at first refused to take him back into Prussian service; by 1815 Clausewitz was chief of staff of the corps that fought at Ligny and Wavre in the Waterloo campaign.
The theory came from a desk. From 1818 to 1830 he was administrative director of the war school, a post with little to do with teaching; the national biography calls it insignificant and not to his liking. He spent the free time writing the book that became On War. The theorists of his day had looked for rules — the geometry of an army's supply base, the advantage of interior lines. Clausewitz answered that war is fought in danger and exhaustion, on reports that are mostly contradictory or false, and he called their sum friction. In a note of July 1827 he wrote that war comes in two kinds: war to overthrow the enemy completely, and war to take some territory along a frontier. He meant to revise the whole book around that distinction and around the tie to politics, and by his own account he finished only the first chapter. Scholars still argue over how to read the formula that war continues politics; some treat it as one side of an argument that the book resolves in what he called a trinity of passion, chance and reason.
He had not meant On War to appear in his lifetime, and had told Marie, half in jest, that she would be the one to publish it. He sealed the manuscript in 1830, when he left the war school, and in March 1831 he became chief of staff to Field Marshal Gneisenau, his fellow reformer; he died of cholera in Breslau that November. Marie brought the book out in three volumes between 1832 and 1834.
Its readers have drawn very different lessons from it, beginning in Prussia. In 1876 the standard German biographical dictionary of the day credited Clausewitz's way of thinking with the victories of 1866 and 1870–71, won by Helmuth von Moltke, chief of the Prussian general staff, although Moltke wanted the conduct of war kept as free of politics as possible. After the First World War the British military writer Basil Liddell Hart blamed Clausewitz for the massed frontal attacks of the Western Front, and in 1993 the British historian John Keegan repeated the charge. In the United States, after the Vietnam War, On War became a key text at the Naval War College in 1976, the Air War College in 1978 and the Army War College in 1981. The 1976 translation by Michael Howard and Peter Paret remains the standard English version.