Seventy years ago, a world went smash. In a sense, we are still waist-deep in debris from (the 20th century is largely debris from) the battle that began at the Somme, July 1, 1916.

A. J. P. Taylor writes that no man in the prime of life in 1914 knew what war between the great powers -- there had not been such a war since 1871 -- would be like. On July 1, it was like this:

Sixty-thousand British soldiers were casualties; 20,000 were killed that day. (Twenty thousand is 40 percent of the eight-year U.S. fatality toll in Vietnam.) By mid-November, when the battle oozed away into the churned mud, the British had suffered 420,000 casualties, the French 200,000 and the Germans about 450,000. The Somme front was 12 miles long. Never was more than eight miles gained.

The war was a calamitous case of new technology overwhelming old tactics. The machine gun suddenly gave decisive advantage to the defense. The old tactic of offense -- slow advances by massed formations -- amounted to trying to wear out machine guns with young men's chests.

On Sept. 15, 1916, a new weapon, born in the fertile brain of Britain's first lord of the admiralty, Winston Churchill, clanked into action: the tank. Twenty-four years later, a German regime made possible by the immobile slaughter of the First World War would send tanks racing across France to Paris.

In 1984 and 1985, we had many observances of 40th anniversaries associated with the winning of the Second World War. Wars are fought by young men, many of whom, 40 years on, linger and remember. Not so 70th anniversaries of wars. However, First World War anniversaries also should be noted because that war was worse and greater. It was worse because it was fought for no purpose as defensible as cleansing Europe of fascism; greater, in that the war's resonances were -- still are -- louder. A consequence of the Second World War was the drawing of the Soviet empire into the middle of Europe. The creation of the Soviet regime was but one evil consequence of the First World War.

The generation that marched to war on both sides in 1914 believed, more serenely than any subsequent generation has, in the inevitability of progress, the beneficence of technology, the wisdom of established authority. That generation went over the top of the trenches, and off a kind of spiritual cliff, at 7:30 a.m., July 1, 1916.

In 1919, the reading public was shocked by the title of a book: ''The First World War.'' Surely, there would not be a second. After the second, the world understood the ruin wrought by the first. The ruin included generalized disrespect for all authority -- political, moral, spiritual -- because so many authorities had sustained the four-year war of attrition. Democratic publics became hospitable to a semipacifism that encouraged the dictators that rose from the rubble of the First World War. In 1922, a British writer said:

''The most bloody defeat in the history of Britain might occur on July 1, 1916, and our press come out bland, with nothing to show that we had not had quite a good day -- a victory, really. Men who lived through the massacre read the stuff open-mouthed. So it comes that each of several million ex-soldiers now reads with that maxim on guard in his mind -- 'You can't believe a word you read.' ''

''Idealism perished at the Somme,'' says Taylor. And what produced this scorched social earth? Artillery, bayonets, bullets. No nuclear weapons were required, a fact worth pondering.

President Carter spoke in his Inaugural Address of ''the elimination of all nuclear weapons.'' President Reagan says his deepest desire is elimination of nuclear weapons. That is a mistaken desire.

To deter with conventional forces the conventional forces of the totally militarized Soviet Union would require permanent conscription of wealth (nuclear weapons are relatively inexpensive) and young men on a scale that no democracy has been willing to suffer other than in wartime. Recently a U.S. senator was musing on the difficulty of explaining to college audiences why nuclear weapons, although now too numerous, are not dispensable. The senator should say: if nuclear weapons were abolished tomorrow, male undergraduates would find themselves headed not for Salomon Brothers and the delights of investment banking, but to Army barracks on Europe's central front for the low-paying trade of deterring Soviet conventional forces.

''Conventional forces.'' The phrase has a soothing sound -- until you remember what conventional forces did 70 years ago. They killed men one by one, but with a cumulative effect that was socially shattering. Nuclear weapons were not required. They are required today for the prevention of battles as ruinous as the Somme.