As a follow up question: was the bayonet charge in "we were soldiers" (the one about la drang) fictional? I've been googling this and Wikipedia says there was at least one charge but doesn't specify if there were bayonets. It seems unlikely they would have bayonets in 1965.
The bayonet was already disfavored as of 1899 as a result of the increased lethality of arms from the 19th century advances in technology, namely the rifled musket, field artillery, field fortifications, the repeating rifle, and (arguably) smokeless powder. Of course, armies continued to equip their soldiers with bayonets after this point, but the case against bayonets is well made by this time.
Here is one perspective from pre-WWI military theorist Ivan Stanislavovich Bloch, an influential Polish economist living in Russia who had the Tsar's ear on military matters, and whose six-volume "The Future of War" describes at length the military tactical and strategic transformations facing the Russian army as of 1899.
To rely simply on the strength of the bayonet in face of modern intensity of fire would be to judge only by the tradition of those times when the bayonet was the last argument in battle. In the Russian army, faith in the bayonet is still sometimes expressed. Among foreign authorities it is no longer met with. The conditions have wholly changed. In former times the result of an infantry battle was thus decided: the combatants advanced upon one another without flinching, exchanged a volley or two, and then rushed upon one another. By such an assault the fate of the battle was quickly decided, the weaker side gave way, and escaped without difficulty if the enemy employed no cavalry. The victors sent two or three volleys after the vanquished, and the battle was over.
The conditions are very different now. Before an attack with the bayonet can be made a zone of murderous fire has first to be passed. Retreat after a repulsed attack upon a fortified position, will be accomplished only after the loss of more than half the attacking force. At such short ranges as will be found in bayonet attacks, almost every rifle bullet will disable one soldier, and often more than one. On a smokeless battlefield the results of such an overthrow will be visible to all. At such close ranges the present covered bullet will penetrate the cranium; but in other parts of the body will have a shattering and tearing effect.^1
Bloch is saying that bayonets were unfavorable because of the modern lethality of arms (which everyone knows), but also emphasizes the demoralizing effect of the modern battlefield, where soldiers are killed at great distances beyond earshot. Elsewhere in Bloch's work he discusses the horrors of not being able to see one's opponent because of the new invention of smokeless powder, and the horrors of being able to see all the casualties on a battlefield, now that war was no longer fought under the haze of gunpowder smoke.
The same factors that made bayonets increasingly unimportant also reduced the effectiveness of cavalry, which in turn made bayonets even less important. Bayonets were employed so heavily in the 18th century partially because of the invention of the ring bayonet in 1690, which for the first time made pikemen unnecessary.^2
It is worth mentioning that the U.S. Army, still as of 20 years ago has a bayonet attachment for the M4 and M16 and sends its soldiers through a bayonet course during Basic Training, a practice that is widely viewed as a waste of time and more for morale-building and fitness than to prepare for bayonet use on the battlefield. This probably sheds some light on why bayonets were still employed in WW1 -- that militaries have a romantic view of them and like keeping them around.^[citation ^needed]
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