ASK ANYBODY for an opinion on the relative merits of the two world wars and the answer is likely to be overwhelmingly adverse to the first. Most people still appear to believe that in 1914 Britain got involved in an unnecessary war - the product of "power politics" without any moral content - whereas in 1939 the country embarked upon a crusade for freedom and democracy. This conventional wisdom is at last coming under challenge, but it remains absurdly dominant. Historians who should know better continue to subscribe to it. As we prepare to mark the 80th anniversary of the end of the First World War, the idea that it was a futile bloodbath is still being aired.

The truth is that on both occasions Britain went to war in defence of its vital interests. Self-preservation was the primary motive for our involvement in both wars; both were equally struggles for national survival. There was also a strong element of idealism in both wars: stronger, actually, in the first than in the second. Since Britain and France (the principal Western allies in the first war) were countries with genuinely free institutions, their fight for survival in the first war can be regarded as a fight for the general cause of freedom. And the same is true of the Commonwealth and the US in the second war.

Part of the reigning mythology is that the second war, unlike the first, was a "people's war". The suggestion is that in 1914 the British people were committed to war by their rulers and then roused by propaganda to a state of mindless patriotism, whereas in 1939 they acted spontaneously and wholeheartedly to oppose the spread of Nazism. In reality, the decision to enter both wars was taken by the British government and parliament of the day, without any formal reference to the British people. In 1914, however, there was profound popular feeling in support of Belgium, whose territory had been invaded and which was putting up brave resistance.

Most members of the Liberal government at the time knew that Britain ought to intervene to prevent the defeat of France, which was Germany's immediate objective, since it was evident that this would be followed by a German hegemony over the European continent - indeed, if Russia were also defeated, over the whole Eurasian land-mass. But there was some division within the government, which the strength of popular indignation about Belgium helped to resolve. In a sense the people did play a part in the decision to go to war in 1914, and neither their patriotism nor their idealism was mindless.

In September 1939 Britain was faced with a German invasion of Poland, a more distant country to which the British government had given a guarantee earlier in the year. As a result, an ultimatum was sent and war with Germany followed, with public acceptance though with markedly less public commitment than in 1914. There was sympathy for the Poles, certainly, though far less intense than the sympathy felt for the Belgians a generation earlier. And, of course, it is a discreditable fact that, whereas the restoration of Belgian independence remained a British war aim and was duly achieved in 1918, in 1945 Polish independence was sacrificed to inter-allied expediency. When the Second World War ended, Poland had to exchange one form of alien tyranny for another. Only the eventual collapse of Soviet power liberated the Poles.

In the first war the army was based on voluntary recruitment until 1916. Next time there was conscription from the word go.

It is true, of course, that Hitler was a more terrible human being than Kaiser Wilhelm II, and the Nazi regime a worse threat to civilisation than Imperial Germany. But it must be remembered that German unity under the Hohenzollerns had been achieved by Bismarck by "blood and iron", in three wars cynically provoked; and that his successors inherited his brutal approach to politics while abandoning his realistically limited aims. The Germany of 1914 may have been less unpleasant than that of 1939, but it was unpleasant enough, not least in its anti-Semitism. (Those who cherish the illusion that Imperial Germany was different in kind, rather than in degree, from Hitler's Germany, should read the work of two German historians, Fritz Fischer and John Rohl.)

Britain in particular was even more threatened by the Kaiser's Germany than by Hitler's, because the former had built a huge fleet deliberately to challenge the sea power upon which our freedom depended. Hitler's fleet was never remotely a match for the Royal Navy in surface strength, even before the crippling losses it suffered in the Norwegian campaign (the only benefit to us from that otherwise deplorable episode). In the supreme crisis of 1940, Britain was acutely threatened from the air, but - even if the Battle of Britain had not been won by the RAF - perhaps not mortally, granted the country's continuing superiority at sea.

Much is made of the horrifying human cost of the first war, and indeed it was horrifying. But the total human cost of the second war was on a vastly larger scale - an estimated 60 million dead compared with about 10 million. British losses on land were admittedly much lower (between a third and a half of the earlier figure), but mainly for the reason that in the second war the British Army was far less heavily engaged. In 1914-18 Britain and France together carried the heaviest burden, and together won the decisive military victory. In the second war the decisive contribution on land was made by the Red Army, before the British (anyway outnumbered by the Americans) re-established a major front in western Europe.

Incidentally, AJP Taylor makes the telling point that at Alamein "the proportion of casualties among men actually engaged was as heavy as on the Somme". But, of course, far fewer men were engaged: the desert war was a side-show compared with the Western front in 1916.

The human cost of the second war was not only far larger; it also involved civilians at least as much as combatants. Nightmarish as the first war was, it was essentially a conflict between fighting men. In the second, civilians were treated as legitimate targets. For two years, for instance, the British war machine was principally directed not against the armed forces of the enemy, but against unarmed civilians. In February 1942 a directive was sent to RAF Bomber Command to the effect that bombing should in future be focused "on the morale of the enemy civil population". Which was the nobler, more idealistic war, so far as Britain was concerned?

The historian Niall Ferguson argues that Britain could safely have stood aside in 1914. The British Empire (he thinks) could have survived, while continental Europe would merely have experienced earlier the sort of unification to which it is now being subjected. This seems to me a doubly perverse and fallacious argument. In one crucial sense the British Empire was not weakened, but rather strengthened, by the First World War. The self-governing dominions played a vital part in it, and came to maturity as a result of it. Yet their effective independence did not lead to disintegration of the British system. The Commonwealth of Nations (apart from Eire) demonstrated its solidarity in 1939. If India had been given dominion status after the first war, as it should have been, I believe that a free India would also freely have entered the war against Nazi Germany.

As for continental Europe, it is grotesque to compare the European Union that has evolved since the last war with the union that would have resulted from a German victory in 1914. The EU that we know is a free association, based upon democracy. The Kaiser's European union would have been imposed by a militaristic power and organised to suit its interests.

The Second World War could have been avoided if the victorious allies had stuck together and resisted the revival of German power. Grave mistakes were made after the victory, but the victory itself should be remembered with gratitude. Britain's perception of the threat that Germany posed, and resolute stand against it, should not be dismissed now as an aberration. The performance of our country in the First World War was magnificent, and deserves eternal honour.