Churchill entered Parliament in 1901 at age 26. In 1904 he left the Conservative Party to join the Liberals, in part out of calculation: the Liberals were the coming party, and in its ranks he soon achieved high office. He became Home Secretary in 1910 and First Lord of the Admiralty in 1911. Thus it was as political head of the Royal Navy at the outbreak of the First World War in 1914 that he stepped onto the world stage.

A passionate believer in the navy's historic strategic role, he immediately committed the Royal Naval Division to an intervention in the Flanders campaign in 1914. Frustrated by the stalemate in Belgium and France that followed, he initiated the Allies' only major effort to outflank the Germans on the Western Front by sending the navy, and later a large force of the army, to the Mediterranean. At Gallipoli in 1915, this Anglo-French force struggled to break the defenses that blocked access to the Black Sea. It was a heroic failure that forced Churchill's resignation and led to his political eclipse.

Churchill was truly a romantic, but also truly a democrat. He had returned to the gold standard, for instance, because he cherished, for romantic reasons, Britain's status as a great financial power. He had opposed limited self-government for India becaus e he cherished, for equally romantic reasons, Britain's imperial history. It was to prove more important that as a democrat, he was disgusted by the rise of totalitarian systems in Europe. In 1935 he warned the House of Commons of the importance not only of "self-preservation but also of the human and the world cause of the preservation of free governments and of Western civilization against the ever advancing sources of authority and despotism." His anti-Bolshevik policies had failed. By espousing anti-N azi policies in his wilderness years between 1933 and 1939, he ensured that when the moment of final confrontation between Britain and Hitler came in 1940, he stood out as the one man in whom the nation could place its trust. He had decried the prewar app easement policies of the Conservative leaders Baldwin and Chamberlain. When Chamberlain lost the confidence of Parliament, Churchill was installed in the premiership.

His was a bleak inheritance. Following the total defeat of France, Britain truly, in his words, "stood alone." It had no substantial allies and, for much of 1940, lay under threat of German invasion and under constant German air attack. He nevertheless re fused Hitler's offers of peace, organized a successful air defense that led to the victory of the Battle of Britain and meanwhile sent most of what remained of the British army, after its escape from the humiliation of Dunkirk, to the Middle East to oppos e Hitler's Italian ally, Mussolini.

Churchill's tendency to conduct strategy by impulse infuriated his advisers. His chief of staff Alan Brooke complained that every day Churchill had 10 ideas, only one of which was good — and he did not know which one. Yet Churchill the romantic showed acute realism in his reaction to Russia's predicament. He reviled communism. Required to accept a communist ally in a struggle against a Nazi enemy, he did so not only willingly but generously. He sent a large proportion of Britain's war production to Russia by Arctic convoys, even at a time when the convoys from America to Britain, which alone spared the country starvation, suffered devastating U-boat attacks.

From the outset of his premiership, Churchill, half American by birth, had rested his hope of ultimate victory in U.S. intervention. He had established a personal relationship with President Roosevelt that he hoped would flower into a war-winning alliance. Roosevelt's reluctance to commit the U.S. beyond an association "short of war" did not dent his optimism. He always hoped events would work his way. The decision by Japan, Hitler's ally, to attack the American Pacific fleet at Pearl Harbor on Dec. 7, 1941, justified his hopes. That evening he confided to himself, "So we had won after all."

America's entry into the Second World War marked the high point of Churchill's statesmanship. Britain, demographically, industrially and financially, had entered the war weaker than either of its eventual allies, the Soviet Union and the U.S. Defeats in 1940 had weakened it further, as had the liquidation of its international investments to fund its early war efforts. During 1942, the prestige Britain had won as Hitler's only enemy allowed Churchill to sustain parity of leadership in the anti-Nazi alliance with Roosevelt and Stalin.

Churchill understandably exulted in the success of the D-day invasion when it came in 1944. By then it was the Russo-American rather than the Anglo-American nexus, however, that dominated the alliance, as he ruefully recognized at the last Big Three conference in February 1945. Shortly afterward he suffered the domestic humiliation of losing the general election and with it the premiership. He was to return to power in 1951 and remain until April 1955, when ill health and visibly failing powers caused him to resign.

It would have been kinder to his reputation had he not returned. He was not an effective peacetime Prime Minister. His name had been made, and he stood unchallengeable, as the greatest of all Britain's war leaders. It was not only his own country, though, that owed him a debt. So too did the world of free men and women to whom he had made a constant and inclusive appeal in his magnificent speeches from embattled Britain in 1940 and 1941. Churchill did not merely hate tyranny, he despised it. The contempt he breathed for dictators — renewed in his Iron Curtain speech at Fulton, Mo., at the outset of the cold war — strengthened the West's faith in the moral superiority of democracy and the inevitability of its triumph.

Historian John Keegan is the defense and military specialist for London's Daily Telegraph.