"And He shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruning hooks: nation shall not lift up sword against nation, neither shall they learn war any more."
-Isaiah, Isaiah 2:4

First criticism: warfare takes many forms, not least of which are the diplomatic and economic. Plowshares are just really BIG swords.

Second criticism: Swords and spears are muy antiquado, and with the development of Piping Hot Death at the Push of a Button!™ no learning is necessary.

Gee, for an omniscient being, G-d sure missed a slice o' the briefing. Omitting the laser death rays sure dates the proclamation. Maybe Muslims are right when they contend that Allah's message has been mangled by all the prophets and priests save Mohammed....

Although I'm not a great Bible reader, this verse (Isaiah 2:4) has always held a special meaning for me. Not just for me. The United Nations has adopted it as one of its unofficial slogans and has a sculpture dedicated to the verse¹ outside its New York headquarters.

There are references to the verse in popular culture from Heal The World by Michael Jackson² (1991) to Claude-Michel Schönberg's musical, Les Misérables³ and even that bastion of high culture, Magic: The Gathering, which includes a card called Swords to Plowshare⁴.

Of course it's not a literal conversion. While it is possible to convert a sword into the business end of a plough, the point of the verse is metaphorical. Instead of directing energy into warmongering and sending your youngest and strongest off to kill and be killed, use that energy to plant fields with grain and prepare the soil for next year's harvest.

It's about paradigm shift. Agriculture and food production instead of killing and bloodshed. Positive and constructive against negative and destructive.

If you really want to convert a sword into a ploughshare, first know what a ploughshare is. It's the curved piece of metal at the bottom of the plough that cuts through the soil and then turns the soil over. Superficially, a sword might make good raw material for a ploughshare. The sword needs to be hard enough to cut through armour and chain mail, but tough enough to avoid breaking when it smashes into an enemy's sword.

A ploughshare needs to be hard enough to avoid wear and abrasion as it cuts through hard. compacted soil, yet tough enough to avoid breaking when it hits a rock.

Metallurgically, however, they are very different. A ploughshare is a low-tech plate of mild steel, whereas a sword is a complex, crafted alloy using different tempering and carburising regimes. It's possible to convert the sword into a ploughshare, but not the other way around.

Those who — like wikipedia⁵ — say the modern equivalent is tanks to tractors are, I somehow think, missing the point.

They will beat their swords into plowshares and their spears into pruning hooks. Nation will not take up sword against nation, nor will they train for war anymore. — Isaiah 2:4 & Micah 4:3