A foreign secretary's pyrotechnic display of mixed signals that sentenced 800,000 British soldiers and sailors to death

A hundred years ago today, an aristocratic fly-fisherman and reluctant politician rose in the Commons and sentenced 800,000 British soldiers and sailors to death. Sir Edward Grey was the most powerful foreign secretary in British history. His intervention the day before Britain's declaration of war on Germany on 4 August 1914 was one of the most important parliamentary speeches ever made.

The House was fearful and confused, still believing that war could be avoided (as the Cabinet had fondly hoped until a few days before). By the end, there were "loud cheers" when Grey suggested that Britain could not "run away" from "obligations of honour".

Later, in the Foreign Office, Grey muttered words for which he is best remembered: "The lamps are going out all over Europe. We shall not see them lit again in our lifetime.''

Grey, whose eyes were failing, was clear-sighted about that at least. The next day the Continent lurched into an exterminatory world war which led, in turn, to Nazism, Stalinism, the Second World War, genocide and the long subjugation of eastern Europe. The "lamps" were not lit again until 1989.

Grey's Commons speech stands up less well to time. It was the most effective speech that he ever made – and a largely dishonest one. It contains two serious misrepresentations of the facts; an emotive red herring; an obfuscation; and finally a disguised admission of Realpolitik.

First mistruth. Grey said: "We have consistently worked with a single mind, with all the earnestness in our power, to preserve peace."

A few weeks earlier, it might still have been possible to stop the war. Britain held the key but did not use it. Or rather, Sir Edward Grey held the key. For 10 years from 1906 to 1916, he travelled abroad only once, but embodied Britain to the outside world in a way that no other foreign secretary has managed before or since. The prime minister, Sir Herbert Asquith, was distracted by the Irish question, cabinet splits and a young mistress.

Grey's behaviour during the July crisis after the assassination of Archduke Franz-Ferdinand was muddled and puzzling. He gave the Germans the impression that Britain wanted to stay out of a war at all costs (as half the Cabinet and most Britons did). But he did not prevent Russian or French war-mongering, based partly on the conviction that Britain would join in.

In the final days of July, Grey put on a pyrotechnic display of mixed signals. He informed the Germans that he could keep Britain and France out of a German-Austrian-Russian war. He implied to Paris that Britain would never let France stand alone.

Second mistruth. Grey told the Commons: "No government and no country has less desire to be involved in war over a dispute with Austria than... France." As he well knew, France had been egging Russia into a showdown with Germany for weeks.

The 3 August speech warns of the threat to the coast of northern France if Britain remained neutral. "My own feeling is that if a foreign fleet… bombarded and battered the undefended coasts of France, we could not stand aside..." This is a red herring. Or rather a red-white-and-blue herring. Germany had pledged not to use its fleet in this way. The passage was intended to stir pride in Britain's mastery of the seas.

Grey turned finally to the "moral" issue of Belgian neutrality. The question was not quite as clear-cut as he portrayed it. A few days earlier, Britain (that is, Grey) refused to say clearly that a German invasion of Belgium would be a casus belli. If German troops merely passed through – and Belgium was just a little bit raped – would Britain declare war? No clear response was given, encouraging the German machinery of mobilisation.

On 29 July, five days before the Commons speech, the Cabinet discussed Belgian neutrality. It decided: "Sir E Grey should be authorised to inform the German and French ambassadors that at this stage we were unable to pledge ourselves in advance, either under all conditions to stand aside or on any conditions to join in."

That was deliberately grey – and typically Grey. By 3 August, ambiguity had been abandoned. At the end of his speech, Sir Edward obliquely admitted the truth. A continental war was now inevitable. Self-interest prevented Britain from watching the "whole of the west of Europe" fall "under the domination of a single power". In other words, "morality" and "honour" were just window-dressing. A recently discovered letter suggests that George V also believed that Realpolitik demanded the defeat of Germany. The king urged Grey the day before the Commons speech to look for pretexts to fight.

One of the great unanswered questions of history is whether this had been Grey's intention all along.

There were two conflicting impulses in the great European capitals that fateful July. The first – dominant in Britain; present in Germany; weak in Austria, France and Russia – was "let us prevent war at all costs".

The second was: "If we don't fight now, the other side will become stronger. We will have to fight later from a losing position." The Germans feared the Russians. The French feared the rising military and commercial power of Germany. So did many Britons, including Sir Edward Grey.

Was Grey, as some believe, a diffident English gentleman who botched the July crisis because he found it difficult to commit himself? Or did he secretly believe war with Germany was inevitable, to be fought sooner rather than later? Most likely, he dithered between the two positions.

A hundred years later, as regards relations with the EU, comparisons are odious – but irresistible. For Britain, now just as then, full-blooded European entanglement is distasteful. Remaining completely politically detached from Europe, as Grey finally recognised, is dangerous. Shilly-shallying between the two approaches is calamitous.