Twenty-five years ago this week, the Argentine dictator, General Leopoldo Galtieri, ordered an invasion of the South Atlantic islands known in Britain, which has used them as a naval base since 1842, as the Falkland Islands, but which Argentina has claimed ever since it gained its independence.

Some 1,800 British civilians became hostages, in effect, on the island. Prime minister of Britain, Margaret Thatcher, responded by dispatching a naval task force to recapture the Falklands. The three-month war cost 253 British lives and 649 Argentine lives. Within days of the Argentine surrender, the Galtieri regime fell, leading ultimately to the restoration of democracy in Argentina.

The Falklands War was seen at the time by many as an anachronism: a colonial war in a post-colonial era. Some members of the Reagan administration, such as the U.N. ambassador, Jeane Kirkpatrick, felt that the priority for America was resisting communist influence in Latin America, and that supporting the British would be disastrous for that cause.

Officially, Washington remained neutral: the secretary of State, Alexander Haig, tried and failed to broker a peace deal before the task force arrived. Meanwhile, the secretary of Defense, Caspar Weinberger, was secretly supplying intelligence and weaponry to the British, while President Reagan privately assured Mrs. Thatcher of his support.

In hindsight, it is clear that Reagan and Weinberger had the right instincts. Most Americans backed a democracy defending its sovereign rights against a tyranny that trampled on the rule of law. Support for America’s British allies was immediate, heartfelt, and vital. It is doubtful whether the British even could have attempted such a daring mission on the other side of the world without American help.

The war began with the sinking of an Argentine cruiser, General Belgrano, by a British nuclear submarine acting on orders from London — an incident that generated controversy for many years afterwards.

The British had only two aircraft carriers to provide air cover for their fleet, while the Argentine air force operated from land. Its sea-skimming French Exocet missiles wrought havoc among the British warships, sinking four and severely damaging several more, in addition to one merchant ship. During the British army landing, two landing crafts were bombed with a heavy loss of life. But the British made up for their lack of numbers by their training, morale, and bravery.

What are the lessons of the Falklands War today? I can think of at least four.

First: don’t leave despotic regimes in any doubt about your readiness to fight. The Falklands War would never have happened if the British had not removed their only naval vessel from the region. HMS Endurance was an Antarctic research ship, not a warship, but her presence had sufficed to deter Argentine aggression. Her absence signalled that Britain was not prepared to defend the Falklands. The foreign secretary of Britain, Peter Carrington, at that time, and his deputy, Humphrey Atkins, were right to resign for their negligence. Mrs. Thatcher herself would have been forced to go to the Falklands if the islands had not been recovered.

Second: once you have embarked on a war against the enemies of freedom, you must persist even in the face of adversity. I remember vividly the shock when the first British destroyer, HMS Sheffield, was sunk by an Exocet. If the Argentines had managed to sink the aircraft carriers Ark Royal and Hermes, the mission would have been aborted. Democracies do not find it easy to justify wars in distant lands, but sometimes they have to be fought — and fought to the end.

Third: rely only on allies who share your cultural, moral, and spiritual values. “In my lifetime all our problems have come from mainland Europe,” Mrs. Thatcher later said, “and all the solutions have come from the English-speaking nations who have kept law-abiding liberty for the future.” The Falkland Islanders would never have been liberated if it had been left to Europe. The Spanish — then a very new democracy — openly supported Argentina, while the French were Argentina’s main arms suppliers. More importantly, Europe then, as now, had no stomach for a fight.

Fourth: without strong leadership, even the noblest cause is doomed. The early 1980s were dark days for democracy. As John O’Sullivan’s brilliant new book, “The President, the Pope and the Prime Minister” reminds us, Mrs. Thatcher was one of only three great champions of liberty in a world full of fools and knaves. Though the Falklands War was not part of the Cold War, let alone the war on terror, it was the crucial test of her mettle. If her nerve had failed her, Reagan would have been left alone at the helm, with incalculable consequences. It was a turning point.

If these lessons had been applied consistently, we would not be in our present predicament. Al Qaeda, Iran, and Syria would not have dared to defy America and Britain if they had been left in no doubt about our staying power in Iraq, the strength of our leadership, and our readiness to respond to aggression with force. Only when those who lead us take these principles to heart will the cause of liberty prevail.