And the bands played on; Somme, by Lyn Macdonald. London: Michael Joseph. (Distributed by Merrimack Publishers' Circle, 458 Boston Street, Topsfield, Mass. 01973.) 500 pp. $24.95.

The Great War - now that was the romantic one. Or so we thought when we were children in England during the early '30s. Every Nov. 11 our school would march us solemnly down to the town square to honor those who had given their lives in that cruelly misnamed ''war to end war.''

The mayor would be there, wearing his great gold chain of office (we all had Flanders poppies in our buttonholes).

Slowly, oh so slowly, the town band would grind out patriotic hymns. ''Proudly they gathered, rank on rank to war,'' we would drone. Then with heads bowed we stood in silent tribute for two endless minutes, ''lest we forget, lest we forget.'' Then the trumpet sound of the last post would prickle our spines and we would vow that ''at the going down of the sun and in the morning we will remember them.''

But then as we grew older something unsettling happened. In the public library we found poems and biographies suggesting that there was nothing great about the Great War. On the contrary, those memorial services could be simply commemorating a fake civilian view of warfare's sacrifice.

In his ''Sherston's Travels'' and moving, cynical war poems, Siegfried Sassoon, speaking out of his own experience, told us what trench warfare was really like. (''Sherston's Progress'' and ''The War Poems of Siegfried Sassoon'' have recently been republished in paperback by Faber & Faber.) He and other military men like Wilfred Owen let us in on the grim secret - war is never romantic, and the last one had reached new depths of sordidness, cruelty, murderous muddle, and boredom. Even if there had been no danger and no particularly revolting kind of killing, thousands of obese bluebottles, endless mud, and rotten food were more than enough to outweigh a few moments of glory.

It gave us schoolgirls reason to be glad we were girls - exempt, we smugly told ourselves, from ever being drafted into the armed forces.

Now that we have the atom bomb to frighten us, it is tempting to settle once again for the heroic version of that old conventional war. As if, to borrow an image from novelist James Hilton (''Goodbye, Mr. Chips,'' ''Lost Horizon''), being smashed by a bomb or sliced by a spade made any difference to the worm. But Lyn Macdonald is unsparing. Her ''Somme'' won't let us muddle history for our comfort, and pretend that non-atomic wars were good wars.

She is, in fact, a superb writer - so good that I would read her if she were writing about pigs. In fact I rather wish she were. As it is, her unemotional, straightforward accounts of the horror of the trenches are more unforgettable than even the TV news photos of Vietnam. But after all, as everyone knows, 1, 000 words are more telling than 1,000 photographs, and I am afraid that some of her stark phrases are going to linger with me for a long time.

There is more to war than horror, however, and more to this book than careful historic accounting of tactics and battle lines (though that is there, too, along with maps and rather muddy photographs showing a landscape marred with ancient battle scars). What lifts this book above most war histories and gives the war a human face is the quotations from the Somme survivors Macdonald tracked down and interviewed. Something of the other side of war - the side that explains, perhaps, why any man ever goes to war - comes through: the exhilaration that near-death brings, the camaraderie and the jokes that go with it, the incredible courage.

But perhaps what makes World War I most poignant to us members of the atomic age is the innocent enthusiasm with which those young, raw troops marched singing into battle as if they were off to a football game. F.W. Beadle, a second lieutenant in the Royal Artillery, remembers seeing a new contingent confront the reality of war for the first time:

''It was an incredible sight, an unbelievable sight, they galloped up with their lances and with pennants flying. Of course they were falling all the way . . . shells were falling all over the place . . . they simply galloped on through all that and horses and men dropping on to the ground . . . it was an absolute rout. A magnificent sight.''

Altogether Britain lost 150,000 men (we miss their grandchildren today). Official records spoke of the ''natural wastage of trench warfare.''

In the early amateur days of the war, whole groups signed on together: ''the golfers'' who played together, civil servants from Cardiff who worked together, boys of the Church Brigade who prayed together. The Earl of Feversham recruited farmers' sons to form the basis of the Yeoman Rifles. (One of them, on duty behind the lines, asked if he saw anything, replied, ''I see a bloody good field of hay going to waste.'')

There was something nonprofessional about B company of the 13th Rifle Brigade sending back to Fortnum & Mason in London for a Christmas meal that started at lunchtime and ended late in the evening.

On the other hand, Fred Darby of the Worcestershire Regiment got into trouble for affixing a stamp onto one of the tough, indestructible army biscuits (the staple food for most soldiers). He addressed it to his wife and wrote on it, ''Your King and country need you and this is how they feed you.''

Pvt. Tom Turrell won the Victoria Cross when he risked his life to half drag and half carry his officer to safety. And such deeds were not rare.

Lieutenant Beadle killed his first enemy soldier: ''I felt nothing. All I felt was relief . . . it was either him or me.''

Sapper A.E. Comer had to persuade himself into battle: ''You grip your rifle and you say, 'Come on, you silly fool, you've got to go.' ''

Along with these quotations are dozens of sickening passages that ought to make it impossible for any thinking person ever to call any war ''glorious.'' It may be necessary, or honorable - but surely never glorious. One comparatively mild comment, the reaction of an old soldier, is enough to convince me of that: ''I've never been able to stand the sight of blood since (the battle). If I prick my finger, I feel sick, even after 65 years and more.''